Point out this concern when getting your exhaust installed, it can easily be worked. Remember these are stock issue manifolds and GM would have never designed them so that it required exhaust unbolting for an oil change. But then GM also chose to use a shield with the stock issue of these cast iron manifolds. I had a crossover installed on my 68. It hangs no futher down than the transmission pan.
Headers are know to warp.
I put the long branch manifolds on my 67 not running yet but thay look good. I presently have headers on the the vehical. I don't have a clue as to what kind. They are long over due for replacement. I have read the recent posts concerning aftermarket headers.