Better measure carefully if trying to put an RB into a pre-67 A-body from
the bottom.
A friend put a 440 in a 64 Valiant around 1975 and it was a very tight fit.
It had regular stock manifolds and was basically wedged in there right
to the inner fenders. Possibbly a set of the small Schumaker Tri-Y's
would help.
After seeing how tight his was, a 383/400 low deck would be a much
better choice and can easily be made into a stroker if neccesary.