Engine placement fore and aft is easy. You need the location that allows your headers to exit the fenders through the original cutouts in each front fender.
Left and right is equally easy. The headers need to be equidistant from the rocker panel on each side.
Driveline angle, as has been said by everyone already, needs to have the pinion and transmission output shafts parallel but not coincident.
Here is the graphical story;
Engines with carburetors historically had a 3˚ tilt down going towards the rear of the car. That means your pinion needs a 3˚ tilt upward.
Easy, peasy ...
Ed