Norm Veber at R & M has a resin small Ford six from the Johan Maverick and Comet kits (144, 170, 200, and 250) which is excellent. Perry's may also do the same engine.
The Lindberg Dodge 330 (I think) and one of the Plymouths have a great slant six engine. One comes with the 3spd manual and the other has an automatic.