Where is the radiator mounted, behind or inside of the rad support? What radiator are you running? I have seen the radiator mounted all different ways in these trucks and it can make a heck of a difference of course.
The radiator originally sat behind the support (nearer the engine) kinda half way, so with the upper bar that goes over the radiator and bolts to each side of the rad support removed and modified you can move the radiator forward about two inches. I have even seen radiators mounted in front of the radiator support completely! This truck had an inline six originally there is plenty of room for a V8. Never did a big block but a small block fits no problem.