The 302 is a windsor block. The 351's could be windsor or cleavland style. Some were "351C's" and others were 351M's. All non-windors had canted valves. So any production non-351C/351M are windsor.
That being said, the early 351 windsors are "said" to have more meat but are 9.49 or something less than 9.5 deck height. The made in Mexico 302 blocks "Hecho en Mexico" seem to be the the ticket for more meat. R302 says 4-bolt race block to me.
There is a good thread on turbomustangs is you do a search on mexican blocks. They are like the early 289 hipo blocks. Only problem with the 289 blocks is that they do not seem to be good for strokers because of the shorter cylinder walls. So the 60's blocks will have a "C" and the 70's will have a "D" in the casting numbers. Unless it is a Boss 302 block. Some replacement Boss 302's had a "Z".
That is where I am confused when you refer to a "R" 302 block that is not a 6010-? FRPP block.
Correct me if I am wrong to keep the thread alive.