Power and manual gates are the same. It would be easier to cut the gate for the tail lights rather than mod the rear quarters to accept different taillights.
It's a modified champion shroud. I think I got a blem as the holes were opposed to what they are now so I riveted a new sheet to the face and cut new holes.
Early a k frames are the same /6 or v8. Your cheapest bet is factory 273 mounts. I have ran a 360, 318 and currently run a 340 with unmodified 273 mounts. Some good bolts, washers and piece of 7/16 all thread is all that is needed to make them work.
I have all the pieces For your v8 conversion. Motor mounts,center link,gas pedal assembly, exhaust manifolds, oil pan,90 degree oil filter adapter, 904 v8 cable trans. The package price on those would be more than the rebuilt slant I offered you.