A few thoughts about the above topic:
1. Rear Disc brakes - I'd love them for my '76, but rebuilt the drums after the Turner rear kit did not fit (in 2011). The Turner's upfront are fine.
2. 200R4 trans was installed in my '76 after the 400 SBC was overhauled and updated in '11. I've been very happy with it and put over 50,000 miles on it since installed, rebuilt last year.
3. Even after overhaul, headwork, 400 still would overheat if in hot weather and stuck in traffic. Went to switch to EFI in '17 and found a cracked head with some cylinder damage necessitating another rebuild, this time with aluminum heads and re-cored radiator seems to have eliminated the problem. Car runs great now. 20-20 hindsight in effect, I might have gone with a 383 crate motor or LS when I got the car in '11 instead of 2 motor rebuilds. The 400 now is no slouch on the road, plenty of HP and Torque, getting around 22 on the road at 70.
4. I had a ''89 convertible for 7 years the same time I had the '76, buying the '89 already resto-modded with a crate 383 SBC, 4L60E AOD trans, Ford 9" rear end, air shocks, 18" and 20" torque thrust wheels. 305 owners, I think the 383 crate is a great drop in answer to upgrade the power in your Avanti. It would go in with little or no modification that an LS likely would need. I do understand a desire to keep the original power plant in the car, a problem I wrestled with in my '76. We also need to consider the originality tag doesn't mean much on any Avanti after Studebaker stopped production.