You don't need to change out the entire rear end if you have either an 8" or 9" rear end. Just swap out the center section (pumpkin). Most likely you have an 8" in it already.
You can take out the pumpkin and have the gears changed out to pretty much any ratio you choose but you need to have it done by a professional or you'll likely wind up with a whining noise. The best thing is hit some of the larger swap meets and you can probably find a complete pumpkin with either of those gears setups for $100 or so. I got one a couple years ago for $60, for a 3.50 setup. I see 8" pumpkins all the time at shows with vendors. If you have new gears put in yours you'll probably spend $300-$400 including the gears. Still cheaper than changing out the entire rear end.
8 and 9 inch centers wont swap between the 2 will they? i can get a 9 inch very cheap but it has drag gears in it, so id like to chage the ratio in my 8 inch until i rebuild my other 302 and then i want to put the 9 inch in with preferably 3.55 gears