Swap 904 to 727?

Why not just find a set of 66 and earlier engine brackets for a V8, they're not that hard to find, and will bolt right to your 78 motor! The K frame is the same for V8 and slant 6 motors in 66 and earlier cars! I think you're going to find that you need to hammer the pinch weld in the tunnel to get the 727 to fit! And a new yoke and shorter driveshaft is also in order!

You also can not use the idler arm from the 69 K frame, it's not the same, but yes in the T-bars!