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It depends how the P and J pups are wired. If it has a single Vol and Tone and a 3-way pickup selector switch (P, P+J. J) then, when in the P position it will be exactly the same as a P bass. On the other hand, if the pickups are wired Vol/Vol/Tone (or Vol/Blend/Tone) then the resistance of the additional pot changes the pickup loading and the solo'd P will sound slightly darker. It's not hugely noticeable, but there is a subtle tonal difference. If you want to retain the original tonal range of the P pickup then wire it with a selector switch.

Fender vintage saddle screws are generally UNC 6-32 x 1/2" (or length to suit)

I'm glad I wasn't there lol!

If no one here can help then there's a long and active thread on TB about Ampeg scroll basses - https://www.talkbass.com/threads/love-of-ampeg-and-other-scroll-basses-part-2.1044767/page-68. Lots of AEB-1 owners on there who may be able to help. Visual comparison with P-bass below might be useful as a guide.

Have you tried swapping the hot/cold wires on the Jazz pup? That's an easy way of checking for a phasing issue. Or try reversing the phase of one pup channel in Logic as in vid below

Ah, how annoying, wonder why they did that!

Excellent!

The Mustang, Bronco and Jaguar SS are all 30" scale so it is possible the JSS neck will fit. The pocket and mounting hole positions look very similar.

The headstock reshape was a brave attempt lol! The SD single coil with the bigger pole pieces is probably quite similar to my Sentell pup. There's something about the sound of a bass with one single-coil that is so open and natural it just feels 'right' to me.

I've spent a lot over the years in my quest for 'the special one' and have owned exotic and beautiful instruments from Zon, Rick Turner and Rob Allen as well as many other expensive-ish basses. More recently I've been playing around with budget basses and have been very pleasantly surprised! My latest one is this Squier Bronco which was professionally modded by the previous owner with a Hipshot bridge, Hipshot tuners, new pots/harness and a Sentell 'Big Sis' handwound pickup (http://www.sentellpickups.net/bass.html). An ebay special for the princely sum of £150. A bit on the steep side for a used Bronco maybe, but it's a little gem! It feels and plays well above it's price point and the Sentell pickup is gloriously rich and sensitive to playing touch. I love it! Anyone else out there with something rather 'special' at the budget end of the GAS richter scale? Post pics if you can!

Ah, sorry I didn't realise the yellow wire joined the two diagonal lugs. Looks like that's just a jumpwire which hardwires the two coils in series then. Which means the two connecting points you need are as in pic below. Give that a try that and see if it works.

This is only an educated guess but looks to me like the linked brass plates are there to ground the pole pieces and the two lugs at either end are hot and cold for each coil with the yellow wire going to the hot terminal as in pic below. This gives you a four-wire hook up with options for wiring the coils in series or parallel. I stand to be corrected!

Not tried it myself but here's a short thread on the subject - http://www.luthiertalk.com/threads/neck-re-profile-to-asymmetrical.360/

From 12th fret to centreline of bridge pickup on my '72 J is 377mm. To front edge of casing is 368mm. This is the early '70s position which is closer to the bridge than standard 60s position.

I had one of those too, took me ages to work out what it was!

Have you tried giving the trussrod a gentle tweak to see if the rattle goes away? A one-way trussrod generally only rattles if it's loose - a small tweak to make sure it's engaged might be worth a try. Also worth checking that the rattle isn't coming from the tuners.

Aha, +1 to the benefits of good luthiery vs mass production!

Radius only makes no difference to string action IF the strings all run parallel and have the same strings spacing at nut and bridge. But in most cases string spacing is wider at the bridge and the strings (particularly the outer strings) then run at a slight angle across the curvature of the fingerboard. The tighter the fingerboard radius, the more pronounced this will be, and this affects how low you can get the action. Conversely the larger the fingerboard radius, the less pronounced this will be and you can achieve a lower action. The difference between the string spacing at nut and bridge is also a determining factor (the more 'splayed' the strings, the more pronounced the hump). Here are some pics to illustrate: Pic 1 - Strings running perfectly parallel with the fingerboard can take a uniformly low action Pic 2 - Strings running at a slight angle have to effectively cross a 'hump' in the fingerboard which requires a higher action to clear Pic 3 - Strings running at any angle across a completely flat board can take a uniformly low action This assumes a fingerboard with a fixed radius (ie, a cylinder with the same radius at either end) which is generally the case. One way to eliminate the 'hump' issue is to have a fingerboard with a compound radius. This follows the curvature of a cone (rather than a cylinder) with a smaller radius as one end and larger radius at the other: This allows a uniformly low action with typically 'splayed' string geometry.

Some input from TB here - https://www.talkbass.com/threads/mighty-mite-vs-allparts-necks.1330074/ Both seem to be pretty well regarded but a few comments about Allparts being a notch up in quality with better fretwork. I thought MightyMite necks were generally cheaper than Allparts? Not tried either myself. I had a Warmoth jazz neck once which was very high quality but the steel reinforcement rods added quite a bit to the weight.

There's a long thread on TB about this with several suggested replacements but all needed drilling - https://www.talkbass.com/threads/sire-v7-5-string-bass-replacement-bridge-tuners.1240782/ From your pic, the bridge and through body holes on the Sire just seem to be about half an inch too far north to stand any chance of intonating the B. A couple of pics on the TB thread have a similar problem with the B so looks to be a basic design fault.