Been following these posts with some interest.
I have a '75 Chev with the 350SB over here in OZ, have an NP4350 (some call it the NP445) close ratio all synchro box out of a Dodge which I kicked around the idea of replacing the Std SM465 granny gear with. It is only a weekend warrior to tool around with, so higher starting first gear won't worry me.
Both have the clutch throwout arm in the same position, just eyeballing it I would think the easiest solution would be to simply add a machined alloy (or steel) spacer block between the original Chevvy bellhousing and the front face of the tranny. NO? I do have the original Dodge B/H but too much ginning around. Easy enough to have a pilot tip extender machined up out of brass stock to suit the Chev flywheel, and have had in the past had clutch discs made up with differing centres (Chev 10 spline--Dodge23 spline, had one made up recently to go from 23 spline Dodge to Perkins 203, but thats another story).
Some alteration to the tailshaft probably needed, but thats straightforward.
Been kicking this around for a while, no gain in final road gearing, both 4th direct 1:1, but the tranny in remarkably good condition, had it sent over from your way for another project. All I had to do was chuck in input/output bearings and seals, fresh oil, came up like a newbie! Would be interested in what you guys think, Dave