the 4.2 is the same bellhousing??
The 97+ 4.2L V6 bell bolt pattern is the same as 4.9/5.0/5.8. The difference is that the 4.2 bell is set up for metric bolts into the engine, so the holes are just
slightly too small for the 7/16" bolts for the 5.0 to pass through. It only takes about a minute (literally) with a drill to correct that. The 2 alignment dowels are the same diameter so no worries there.
For the clutch/pressure plate, 96 5.0L F150 11" or equivalent.
Your 91 5.0 engine needs a 50 oz-in imbalance flywheel and the R2 needs a 164T ring gear - this combination was also used on the F150, 96 and several years earlier.
Get a manual truck starter. Most later automatic truck starters and many Mustang starters won't work with the R2. Again, the 96 5.0 150 manual or equivalent is a good source.
Use a block plate for a 302 with 164T flywheel. Some early Ford/Windsor cars or, need I say it, the safe bet is the 96 F150 manual part or whatever crosses to it.
also what about the master cyl? is it going to work the slave correctly?
If your current transmission has the dreaded internal slave cylinder, then chances are that the R2 is compatible with your master and line. Some of the eighties Rangers used an external slave with a larger bore master but I don't think those were in use as late as 87. You'll need to do some research.
could just the top cover be switched as i allready have have a trans from a 300ci truck.
I remember someone posting that it is possible to swap covers between early and late R2's but I don't know if it's true or not.