V8 swap info

Thoughts.........The original 3 speed is the smaller lighter version, I would not consider it. Bellhousings must be drilled to match the trans "generally" but some are dual pattern and will take either 3 or 4 speed

Four speed: Do you want, or make sure that it is / is not an OD unit. Quickets way to tell if it has the shifter properly installed is to look at the front shift lever. If it points down, it's an OD. Alternately, you can wrench that shaft CW which puts the box into 3rd gear if straight through 4 speed, or OD if an OD box. Then turn the input shaft 1 exact turn and watch the main shaft. If it turns LESS than 1 turn, it's a "straight through" 4 speed. If it turns MORE than 1 turn, it's an OD

I have a magnum 5.9 in a truck that WAS NEVER DRILLED at all for a bushing. If you use the later, larger "Jeep" style bushing that fits the converter register, you can drill the crank hole "sloppy" as it is not used to support the bushing, but measure for depth

I see nothing wrong with a 904. You can either hog out the one hole for the converter, as the 904 converter has a non symmetrical bolt pattern, or use the older flex plate, if you are not using EFI. The magnum converters are symmetrically drilled. The difference is, that the weights for balancing the assy are on the converter for the 904, and on the later Mag flex plate for Mag engines.

I AM NOT SURE about balance on a 5.2. May / may not be neutral balance

You are aware? I hope? that you need a small block V8 bell bolt pattern, you cannot use a slant 904