[1project2many]

I just spent half an hour typing a response and you go and change course while I'm typing.

AFAIK the bellhousing for that trans is an integral, cast in part. That trans is intended to work with a rear crossmember.

Distributor clearance is set by engine position, of course, which will be determined by where you set the engine. A simple procedure to get everything close to right is to get a bellhousing from the 55 - 63 period. It can be car, truck, or aftermarket. Attach your engine to the bellhousing and set it in place in the truck. Fabricate front mounts. Remove engine, separate OEM style bellhousing, then install NV3500 and re-install in truck. Fabricate rear crossmember.

NV3500 issues: No speedometer cable connection. Longer than stock trans (may require driveshaft modifications). Shifter is positioned rearward from stock. Rear yoke may fit turbo 400 applications. Sensitive to poor maintenance. Hydraulic clutch will require hydraulic master cylinder installation.

I think you're looking for an "overdrive" to keep RPM down on the highway. But the answer with the least work in cost and effort is likely to be a little different. A 350 makes plenty of low end power and I don't think you're going to be hauling heavy cargo. If an automatic's out of the question then consider using an SM465 four speed trans, an easy bolt in with the 55-63 bellhousing, and a numerically lower rear gear plus a narrow 16" tire (215/75R16). IIRC a Nova or Camaro rear from the 70's through 81 can be installed easily. They can be found with 2.73 and 3.08 gears which will bring rpm down on the highway, just like a 5 speed. The more modifying you do the harder it is to get the right parts when things break.

One other note. Make sure your block has provisions for front mounts if that's the route you're going. Some don't.