Does it matter how close the cap is to the hood . I got the c body radiator that went with my 383. From 70 Newport if I hold it I can close the hood but there's not much space between the two measured it to the cap its 24" tall
It's not so much how close it is now, but if it'll dent the hood when it gets closed.
Do you have any way of determining the clearance when everything is in place where it will be living?
1/2 inch should be ok.
Don't forget that you should have something between the rad and the support to keep it from wearing holes in the bottom of the rad.