there are other ways to do it. your carb base on the manifold should be level from frt to back. or the c/shaft centreline should run fairly parallel to the pinion centreline. it doesn't mean that they have to be inline with one another, just that they are parallel

it would be a fixed point if one is using the stk type powerglide crossmember to install. it you are installing something else, then you have to find that point
the motor mounts are fixed but they are a ways forward
ron