My car is a 72 witch came with the 307 and 350 trans, I have a 383 in it now with a 400 trans. Ive changed out almost everything (motor mounts, trans mount, crossmember,ect..) Had lots of trouble with crossmember and had to do some mods to make it line up.Now motor and trans look to be straight, but when I went to install headers they hit on the floorpan. Headers are correct part number, checked. Miss lining up by 1 to 1.5 inches.Motor is 350 block with victor jr heads, could it be heads?

i cant say i know if its the heads, but i can say that i have heard other members having trouble with headers hitting the floors. Ultimately, it *could* be anything that adjusts the height of the engine from the motor mounts to the heads. This does not appear to be a rare problem, though.

I assume these are LT's?
Hrm...and I was gunna buy LT's for my 72.
What kind of header and how big are the primary/collector tubes?

Lots of possibilities here. Just a few:
motor mounts too tall
transmission mount / crossmember raising tranny too high
worn, compressed subframe body mounts (lowering the body and floorpans referable to the subframe and motor mounts)

Thanks everybody, and if anyone has more info on this problem please tell.

What kind of "trouble" did you have with your transmission mount? Should have been a snap to replace stock one. Maybe 7T3Z is on to something here. You may have raised your tranny too high in "modifying" your crossmember???

When you say "now it sits straight" might be the problem
I changed from a 350-400
Cross member went in with no modification
Spent 6 months trying to figure out why my big block wasn't centered in the engine bay.
Found out it's not supposed to.

Is it an after market mount
Try putting the mount in it's original position
Headers should fit without any mods

Just for the record, Im starting to think it may be all the new mounts and bushings(motor,trans,body).All mounts and bushings are energy suspension if that means a thing.The whole trans mount issue was the crossmember bolt holes would not line up and I had to notch them all, no up or down adjustment. The motor and trans look to be sitting just right so I dunno. If anyone knows good aftermarket performance headers that clear floorpans, please tell. Looking at Patriot Clippster Headers anyone know about these?

By the way headers are Hooker Competion, and the crossmember is a cheap one from D&R Classics looks like stock. Anyone know of aftermarket crossmember maby tubular.