got a set of 4th gen headers cheap anybody try em on a 3rd gen?

Wont work. The sbc head have the exhaust holes like. O OO O and lsx is O O O O.
However if its a lt1 header it will have the exhaust holes as classic sbc, however wont fit due to the radical frame differences.

if they're shorties, should fit exactly the same as the factory manifolds. if they're long tubes, been discussed quite a bit already, not a chance they'll fit.