What is the best motor to put in a 1956 Chevy Bel Air?

Depends if you want the car to be as close as stock as it can be or if you are just building it to drive. If you are going stock then you will have to install the original engine type which will be either a 235 cubic inch 6 cylinder or 265 cubic inch V8. The serial# plate will tell you which engine was installed from the factory. It will look like this.

V C 56 S 1001

V = V8 (no letter here means it was a 6 cylinder)

C = BelAir

56 = Year

S = Assembly Plant S=St. Louis. A=Atlanta, B=Baltimore, F=Flint, Mich, J=Janesville, Wisc, K=Kansas City, L=Los Angeles, N=Norwood, Ohio, O=Oakland, T=Terrytown, N.Y.

1001= Serial #

If you are not looking to go stock then a small block 350 Chevy Crate engine would be the way to go. Not that expensive and comes with a 3/36 warranty. You can even get one with fuel injection. Contact your local Chevy dealer for more info.