A 1/4 or 1/5 scale car with a V8 will be a "Do-It-Yourself" project. No Ready to Run V8 car kits out there that I know of.
And get ready to spend some serious money once you begin. Those Conley V8's are in the thousands of dollars. Never mind the car.
They charge $5,695 for the base engine with the following add-ons:
Exhaust manifolds, cast in 303 stainless steel......................$ 279.00
Exhaust manifolds, cast in 303 stainless steel - polished ....$ 559.00
Painted Valve covers and intake manifold.............................$ 125.00
Supercharger installed and tested..........................................$1 ,695.00
High duration camshaft (installed at factory only)................$ 279.00
Man! I could take that money and put a very healthy engine in my full scale S-10 pickup for that kind of bread!