The lower bolts may be a bit of an issue but using the correct technique, will solve.
Novak says:

"While the Muncie 420 can be used with any GM engine having the "regular" GM transmission bolt pattern, this transmission installs more easily with the open bottom Chevrolet bellhousings because the lower holes in the transmission case are threaded rather than drilled for clearance. The threaded hole case cannot be modified, so the lower attaching bolts must be installed from inside the bellhousing.

This is minorly inconvenient on full circle (one piece) bellhousings such as Buick V6
and 1962 and newer Chevrolet V8. The lower bellhousing holes must be drilled for bolt
clearance (17/32" drill) and the clutch release arm, bearing, and bellhousing must be
assembled to the transmission and then assembled as a unit to the engine. While this may
not be convenient, it is not difficult and the results are worthwhile.

When using later GM aluminum bellhousings with the SM420, note that the transmission top cover casting protrudes into the bellhousing rear face and machining for clearance may be necessary."

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn