I have a standard 1970 Ford 302 that needs to be rebuilt. I was wondering if converting the block to a roller block is worth while or should I just go out and buy a late 80's block that is already a roller? Would it be cheaper to get a set of late model heads with hardened valve seats or have the work done on the old ones?  

The late model engine and heads are cheaper by a long shot. Also,due to low
tension rings and fuel injection,the late model engines typically only need
new rings and bearings during a rebuild.  

When are you defining "late model"?  I've got an '83 Bronco w/ 302, but it's a rust bucket.  Recently picked up a clean '86 with a blown motor.  I plan on swapping the 83 block into the 86 with the 86 EFI and emissions crap (live in CA). Is there any drawback to using the 83 block, or should I spend the time trying to rebuild the 86 block?  It's gonna be a stock, daily driven, cheap as possible build - thanks for your input.