If it was my project I would have the core support mounted to the frame. Then I'd get the engine in so the back of the head is close but not touching the firewall. I would also get the intake manifold on the engine so I could check the angle that the carb will be at with the truck as ride height. You can also mount the radiator to check the location for the fan. You should also consider the height the engine is in the frame so you have proper ground clearance and not to high so your hood will have clearance. Lots of stuff to check.