I just want to make sure I've got a decent understanding based on the searching and reading I've been doing.

From what I've gathered you can build a hybrid engine using 12A plates with 13b housings as long as you match all counterweights to their respective parts and a budget for a good engine balancing to make sure there are no extra vibrations(you can get away without but wouldn't hurt) and of source use the correct 13b parts as needed.

So do I have the right basic idea tuning not considered? Or did I miss something?

 

YES! We have a winner.

 

Right, match the counter weights to your rotating assembly, the e-shaft doesn't count in most cases, find the right intake, you're good to go.

 

Alright, thanks, glad to know the reading has been paying off some.

 

One thing to remember, all 12A's and the pre-86 13B's have the coolant seal grooves in the rotor housings, while the 86-up 13B has these grooves in the side housings (AKA irons).
So you need 85 or older 13B rotor housings, or you will have no place for the coolant seals to fit.
Rotating assembly is pretty much up to you, as long as the rotors and counterweights (or stock flywheel) are from the same series engine.