EFI conversion cost depends entirely on how much of the system you are willing to scrounge.
2 of the TBI units off mid '80s domestic econoboxes hooked to a megasquirt contoller seems the most cost effective for what you already have.
However if you would prefer carbs, Langdon has a dual 2 barrel Carter Weber setup that should work for you (last I heard he still had new carbs for that).
