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.
If you want to use that 3.8 (sweet motor- friend has it in his Jeep), you'll need a tranny from a 2.8L S-10.