Just went through this for my 65 Valiant. My car is fairly high HP, would do it differently if you're running a mild 273. No one makes a high performance 9-1/2" or 10" borg & beck pressure plate (Mcleod used to). Luk makes one for cheap but my understanding is the quality is not there for HP applications, might be OK for a mild 273 or slant 6.
Got a billet 122 tooth flywheel from Wilcap. Had a nice original 9-1/2" pressure plate rebuilt by McLeod & bought their very last 9-1/4" 23-spline Rev-lok steet disc which is a mild sintered iron unit that has a marcel carrier. This should hold about 400 ft-lbs torque. McLeod is discontinuing most of their non-high-volume stuff & 9-1/2" mopar stuff will be a casualty. For a backup I also bought a 10" 23-spline Rev-lok disc from Mcleod. They still have a few of these left but when they're gone they will be discontinued as well. The 10" will not fit the 9-1/2" stock pressure plate however they did make a very heavy duty truck 10" borg & beck some time ago that will fit the 122 tooth flywheel (10-5/8" attachment bolt circle). By chance I found one of these on ebay new for $65 shipped but they don't seem to come up a lot. The 10" should hold a lot of torque, probobly 500 ft-lbs or more with the Rev-lok. You could also use a 10" diaphragm (used on Fords) but this clutch has a different bolt arrangement than the B&B so the flywheel would have to be drilled; also there may be clearance issues with the bell. The nice thing about the sintered iron discs is they don't require much force to operate. A buddy has a 700+ HP roadracer with 3 tiny sintered iron discs (Tilton) & the clutch action is lighter than a Suburu with great modulation.
There are pluses & minuses doing it this way. As far as weight the 122 tooth flywheels & clutches are way lighter than the 130 tooth setup. The original iron bellhousing is about 12 lbs heaier than the later aluminum setup but is probably stronger. As far as cost it actually would have been pricier for me to switch to the 10.5 aluminum setup since I would have to buy all the parts & would have had to go hydraulic clutch. You would have to relocate a pipe from the Doug's headers which I'm using if you wanted to use any type of mechanical linkage.
My advice for a mild application would be to get your original clutch rebuilt by a good outfit. Original high quality 9-1/2" pressure plates are an endangered species.
