Alan:
Can't answer all your questions, but I do have more than a passing interest in this cartridge and can offer a few thoughts.

If you are prepared to drop the velocity a little, you are talking about the ol' .577/90/650 load. According to the 1926 ICI catalogue that load managed 1950 fps (5500 ftlbs ME) in a 28-inch barrel with less than 10 tons pressure.

The 1902/3 Kynoch catalogue gives it 2000 fps from a 30-inch barrel using the 3-inch case, at 10 tons even.

Pondoro Taylor waxes lyrical about the .577/75/650 load, which is listed by Kynoch as the top 'nitro-for-black' load in the 3-inch case, achieving 1850 fps (probably also from a 30-inch barrel) at under 10 tons.

Since none of these loads exceed 10 tons, the chamber-wall measurements from any .577 BP Express rifle should be good enough, with a generous margin for error on account of modern steels in a newly-built gun.

Weight will probably need to be 11 lbs minimum for the 90-grain (cordite equivalent) load, Taylor reckons 10.5 lbs would be OK for the 75-grain equivalent load. I think he's right. My Greener .577 conversion is regulated for the 65-grain load, and although a little stout is very comfortable to shoot at 10.5 lbs.

I have stated before that a major advantage of the 2 3/4 case IMHO is that no-one can ever load a full nitro .577/100/750 cartridge at 14 tons pressure and spontaneously disassemble your handiwork! True, pressure will probably be slightly lower with the 3-inch case, but you will need more wads. Since you will need some wads or filler in the 2 3/4 case anyway, this probably doesn't mean much. Your call.

Every now and then a mischevious little demon whispers in my ear to try heavier loads in my Greener, but so far I have resisted, which has no doubt contributed to the longevity of both the gun and the shooter I'm sure!