Yes. 400 Super Dream.
No studs were damaged but I did a nifty head and barrels off job with the engine still in the frame. Welded some pieces of an extension bar together to get at the big end nuts without needing to split the engine. All done and back together in an evening. It's not how I'd do it these days but I was an apprentice and more willing to take risks then.
Oh yes... Like you said, EZ outs (screw/bolt extractors) may be the answer but you have to be very steady with the tiny drill bit to get the pilot hole in the centre. The smallest extractor is usually good for screw shanks of 3 to 5mm diameter. You will need a tap wrench to use one.