Depends on what you're going to do with it . . . Although I haven't built a gasser or other door car for over 40 years, NHRA still has some rules pertaining to engine setback for certain classes.
You'd probably be better off reading the rule book pertaining to the class(es) you want to run. As an example, I think (Don't hold me to it) S/G has a 10% (of the wheelbase) setback using the centerline of the spindle and the #1 spark plug as the data points.
Certain classes have certain dimensions and it shouldn't really be an arbitrary decision . .