This sounds like a great project. I've recently changed my tenor guitar to 'Chicago' tuning - equivalent to the top 4 of a guitar (or baritone uke).
As the scale length is shorter I used heavier strings than I would for a normal six string .
You can get a good idea of which strings should work well with the tool at http://chordgen.rattree.co.uk/tensiontool.php
eg for a balanced eight string with a 15" scale length (like a tenor uke) using steel strings:
The headstock on the thunderbird vintage pro is a good bit longer than other thunderbirds I've owned.
The tuners on Bigthumb's Precision are Hipshot HB2s https://hipshotproducts.com/collections/bass-tuning/products/hb2-bass-tuning-machine
I have a set with lollipop paddles on the fretless bitzer precision I built.
The HB7s have a smaller plate and are the ones made to fit on Mexican Fenders.
This is the 60s-style Gotoh non-reverse GB640 res-o-lite set on one of my thunderbirds. I used to have an MIJ 62 reissue precision - it always seemed a bit strange to me that it was fitted with 70s style tuners rather than ones that looked like these.
The tuners bigthumb want to remove are the type you want Pirellithecat - maybe you can do a deal?
Bigthumb, you will need to drill extra holes to go from 70s to 60s style tuners as the holes aren't the same place on both. Also the pure vintage and others made on the same way have folded over tabs on the back which will prevent the tuners fitting flush to the back of the headstock unless you make shallow holes/dimples to accomodate them.
For 60s style tuners I'm a big fan of Gotoh GB640 res-o-lites - I have fitted these to both of my Epiphone Vintage Pro Thunderbirds where they look fantastic. Although they are more expensive than most, these are lightweight and flat on the back of the plate. Unlike other 60s-style tuners they are available in non-reverse wind and reverse wind versions - I use the non-reverse version so they work like the tuners on my other basses.
It looks to me as if several design decisions were made not for practical or aestheic reasons, but to be different. Nothing wrong with that if it floats your boat, but not for me.
I have a CIJ one exactly like this as well - they're great basses with s lovely slim neck and punchy sound which records really well. I doubt I'll ever part with mine.
Judging by the look of these, this young builder is perhaps exhibiting the same Dunning-Kruger effect as a certain guitar improver who is not to be named (unless in biro on a headstock) if he thinks these are £300 instruments.
I had one of these. Mine was made in 1990 and had the serial number in gold below the 'made in Japan' as shown, but it had a plain neck plate without the lettering.
The one shown has non-original pickguard, bridge, pickup and knobs.