[77 TRASHCAN]

Although I have NOT performed this swap...
My thoughts would be of using the later 70's type mounts. I talking about the clam shell mounts that bolt to the frame. Chevrolets used them also. The Chevrolets used what looked like a box, open on one end, it had holes front and rear and sat down over the clam shell mount with the typical 6" or so, long bolt joining them together.
You could lower the engine and trans together into engine bay with both parts of the mounts bolted to the engine. Level the engine from side to side.
Not sure at all how this might or might NOT work. Some measuring of frame and mount spacing beforehand would give you an idea if you are close.
Spacers might need to be made if frame is too wide, can't imagine frame being too narrow on one of these car's, another thing to measure before you got too far along.
I'm sure you could use small block brackets on this deal, they are plentiful, came on tons of vehicles. Cars and pickups might be different, if so getting a set of each might give you some adjustability for installation. Buy the frame clamshell mounts new, they are cheap.

Do you already have the 454? If so, what year?
I too favor a Pontiac engine, it's your car. Swapping a 389 for a 454 would weigh fairly close to the same.
Have fun!