Downey and advance adapters should both have bellhousings to run a toy tranny behind the 4.3.  I'd love to stay toy, but the 3.4 is more of a peaky engine, whereas the 4.3 is more of a torquer.  I'm personally planning on running a buick 3.8 at some point because I already have it, but it's also a good in between engine and fits in an early toy much nicer.