I have just put a series 1 Rx-7 5 speed into an Rx-2 (manual transmission, 1971).
I have used the standard 2 piece driveshaft.
There were no clearance problems because the Rx-7 tranny is longer:

That puts the engine more to the front:

The starter motor does not pop straight in, I had to turn it around first, but no hammering required. The clutch slave fits too although it's a tight fit:

A few pictures from underneath:


The engine is too far to the front to allow the fan to be mounted so I am now moving the radiator about 10mm to the front.

To be continued...

 

I went from a '71-'73 12A to a '74 13B and REPU tranny in my MG Midget. I know what you're talking about as far as fitting everything goes, however, I kept the front of my engine at the same location and had the driveshaft shortened by 60mm (40 for the tranny difference, and 20 for the 12A to 13B difference). Speaking of millimeters, I had only a couple to spare when installing my starter. My shifter is further back too.

 

I know it isn't good for the car balance to move everything forward. I couldn't find anyone here (Belgium, Europe) to modify the driveshaft so I took the other path...
It does have some advantages though: perfect shifter location, enough room for starter/slave cyl.