I have found that an engine test stand is an ongoing project. Since I got mine going, I have added a tachometer, O2 meter, oil temperature gauge, and dual (6 and 12) voltage capability, among other changes and improvements. When I built mine I used the works from an old 3-wheel engine stand to provide a universal rear mount, and settled on a standard '41-'48 Ford front mount arrangement. A flathead (early or late with truck pumps) bolts right on and I have the Hurst-style front mount adapters for the SBC, Olds Rocket, and early hemi that are up next. The front mounts can be fixed to the stand, but I made the rear adjustable so it can slide back and forth to account for differences in length. This is in case I ever run into the 320' Roadmaster or 356' Packard I've been thinking about.
Another thing about wheel (caster) size. The bigger the better, but at least make sure that they are big enough so the frame of the stand is high enough off the floor so your "Cherry Picker" will slide under them. Makes life a lot easier.