Unless GM has changed the bolt pattern on the later-model 4.3's, it would have same pattern as a 90 deg Small Block Chevy V8 (i.e., incompatible with a 60 deg V6 tranny).

The 4L30E has the same bolt pattern as a 60 deg GM FWD V6 (i.e., starter on the driver's side if it were mounted in the vehicle fore-to-aft instead of side-to-side).

It does have a removable bell housing so theoretically you might find that a bell housing from a 90 deg V8 or V6 might bolt up. This would greatly simplify the 'shoehorning' in of a 4.3.

Just a thought, maybe Jerry L. or other swappers can confirm......ed