Tricordati, I moved your post to a new thread. Hopefully you get more responses this way. Plus, the thread you posted to was quite dead.
FWIW, if you want to put P225/75R15s on you stock rims I suggest you inquire at a tire shop. You can also do some research on Tire Rack (shameless advertising).
I bought P235/75R15s to go on my 71 98. I found that these were closest to the J78s that were originally on the car. I used General HTS radials. They are good tires without an agressive tread. I think you'll find that radial tires in the sizes we need for our land yachts and luxo barges are generally for light trucks and SUVs. I rejected numerous tires in the right size because they had an agressive off-road style tread.
1971 Oldsmobile 98, 455 (of course)
2011 Ford Flex, 3.5L Duratec V6
2003 Ford F150, 5.4L Triton V8