I guess one of the possibilties is that the rad position has been changed to accomodate the 261 and an older engine might end up with quite a gap between the fan and the rad. Some photos would be really helpful.

1951 GMC 1 Ton Flatbed -- It is finally on the road and what a great time I have driving it!
1951 1 Ton Completed

My Chevy Master 4 Door is on the Road!