The easiest way is to figure out where you want the center of the wheels in the wheelwells. Once you do that, measure from that line straight back to where the firewall is. having the measurement from the firewall to the center of the front tires, you can then locate your engine with a couple inches of space between the back of the engine and where the firewall will be.
On these frame swaps we always have the chassis complete with the engine and transmission mounted and driveshaft in, before we begin to mount the cab.
