I've seen Ross Gibson Engines selling around $20. to $25. each. I
personally would try to work out something cheaper, but that's just me.
Even if I start with the engine/tranny that came with the kit, I'll
scrounge through my spare parts to try and doctor it up. I'll even
spend the time to paint little details like the intake bolts, or run
some vacuum lines and such. Adding extra details somewhat hides the
fact that the original lacked some.
Anyways, sorry for trying to steer you away from ordering shoehorn in motors.
I believe his engines are made of resin and probably based on just a
certain few stock kit sizes. Contacting him might help get a better
explaination regarding his products. But I think you'll be stuck
customizing the chassis to get them to fit in most case's. Kind of like
trying to put a 454 cu.in. in a Vega, a bit of alteration is necessary.
Wish I could have been of more help.