I ran a 455 in a 84 monte carlo. Same frame as far as i would think as a regal. Used poston frame pads for a 67 chassis. Ran mt super scavenger headers. This was many years ago but i dont remember havin to do any major massaging to make them work.
Ta's work pretty well but you will have to "massage" a few of the tubes and may have to grind on the upper control arms a little bit. Make sure you have an extra set of hands because you have to put the headers in the engine compartment first and then drop the engine in between them.
I used headers for a 455 into a 67. I had to dimple ever so slightly the passenger side, and relocate the brake lines on that side as the header was making the line very hot. I had an 83, I believed I used 67 engine mounts as well.
