As for the input collar, you could also try Zumbrota Bearing and Gear 1-507-732-7994 M-F 8-5 central time ask for Craig. They also sell on Ebay under zbag648. They actually have a Muncie collar on Ebay right now for $18, assuming it uses the same input bearing as your tranny...
Considering the project you are building, and the fact that you are not trying to use a 37-54 car or 37-47 truck bellhousing, then the adapter from Buffalo is irrelevant.
Once you have changed over to the larger input collar, any 48-62 truck or 55-62 car 6cyl bellhousing can be used, but some are better than others depending on how you are going to use it. The differences are in the motor mounts, foot starter, and fork direction.
55-62 car has no provision for a foot starter, and I'm not certain what it uses for motor mounts. My guess is that the 55-57 has mounts, but that the 58-62 doesn't???
48-53 truck has foot starter and "square" corners for a saddle type mount.
54-57 has foot starter and angled corner mounts.
58-59 no more foot starter, still angled mounts.
60-62 angled mounts and a hydraulic slave cylinder activated clutch fork on the pass. side.
If anyone has more to add (or correct) feel free...
The 60-62 one that you have could be the one to use, especially if you are using swing pedals. Hook up a slave to the pedal, and just run a line down to the other one on the bellhousing, and you're all set. No mechanical linkage to screw with. Of course, the others can be converted to hyraulic if you prefer. A hydraulic throwout bearing setup could also be used. It all depends on what you want. Mechanical linkage is also doable, but may be more work, again it depends on what you are using for pedals, etc...
