headers are hitting against frame on my 70 chevelle changed from small block to big block headers seem to big any ideas on a quick fix or should i take them back and try some different ones

Where are they hitting, frame or control arms? It's a fairly common problem to hit the control arms. If that is the problem you can 'ding' in the headers using a socket and BFH. Just enough so they don't hit.

I had this issue with my headers I welded a 3/4 piece of steel bar stock to the end of a chisel for my air chisel so that it resembled a T and was able to ding the headers without haveing to completely remove them which would be so difficult that I decided they are only going to come off with a sawsall, I put them in at the same time I installed the engine and thier a tight fit. It worked quite well but its very loud use ear plugs if you do it.

Make sure the eng. mount's are not sagging and making eng. lower than it should be.I usualy use solid mount's so the eng.don't move around.Sometime's you have to do some small mod's like mentioned before to make them work.Good luck.