is it safe to do a h/c/i swap on a higher mileage engine

i herd bad things if the compression is not correct

 

Condition the lower end is more important than the mileage.

 

thx five7kid....any more info guys?

 

There are 2 considerations. They are related. Static compression ratio, and cranking compression. As you mentioned a higher mileage short-block, I'm assuming dished pistons. If so, choose heads with smaller combustion chambers to keep your compression up.
Cranking compression is what really matters, it should be around 190 psi, but anywhere between 140 and 210 is tolerable on the street.
Big cams and higher elevation tend to reduce your cranking compression, which is why big cams call for increased static compression ratio, while smaller cams tend to increase cranking compression. This is mostly about intake closing, but partly about overlap.
If you're putting aluminum heads on a 350, look for heads with 65cc or smaller combustion chambers. This'll keep you above 9.5:1, assuming proper piston-to-head clearance, and under 10.5:1 with aluminum heads will run fine on pump gas with the larger emissions-legal cams. Any cam with no overlap at 0.050" will pass an emissions test.
If it's stock GM pistons, they should be about 0.025" below the decks. If so, use FelPro steelshim head gaskets. Even for a 305.

 

great info atilla thx