have you installed your radiatior? and is your body bolted to frame?
If so just center the motor between frame rails , determine the height for tranny and drive shaft angle and make sure you have room for a fan behind radiator, and you might have to cut the firewall and move it back a tad to get the motor fitting , then just build motor mounts and weld them to the frame.
Its how i did my 37.

SprayTech