I'll see what I can do here.
As far as a clutch, you're probably going to want a 10.5" for the 130 tooth flywheel.
The easiest way to go with the swap is to get an 833 OD out of a '75+ A body or an F body, that will have the "right" tailshaft and shifter mounting location. But, if you're modifying the driveshaft anyway, you can also make a long tailshaft OD work too, and they're a lot easier to come by as they came in small trucks up to 1987. Big Block Dart has an article on what that takes...
http://www.bigblockdart.com/techpages/abconv.shtml
You can see in the article above that this isn't accurate. The 833 OD's came in light trucks from 1975-1987, and those have the long tails and B/E shifter mounts. The Ebody shift mount usually isn't drilled, the trucks used the B body location. Also, the trucks have a .71 4th gear vs the .73 in the cars. There's a ton of info on the 833 OD in this article over at slantsix.org
http://www.slantsix.org/articles/4-speeds/ODA833fourspeed1.htm
You'll need one of the later bell's with the 5.125" bearing retainer opening. A bell with a 4.80" opening will work if you have the input bearing retainer on the OD turned down, or you can also buy an "adaptor" input bearing retainer that has the larger 4.16" bolt pattern and a 4.80" outer diameter.
Can't help you with the hydraulic clutch, never done it.
As far as upgrading the aluminum case, I don't think its necessary for a 273 as long as you find an 833 OD that's in good condition. I know folks that have run unaltered 833 OD's behind 440's on the street without issue. The upgrade that's most commonly done is steel bushings for the countershaft. It's not that hard, there's an article on it somewhere that I can't find at the moment. Basically you just machine the case and insert some steel bushings to keep the case from wearing out at the ends of the countershaft.
