I used an original style Must II crossmember in mine when we did it about 15 years ago and it cleared my 250 motor OK but am not sure about the GMC. I believe that Chev's of the 40's and R.B's Obsolete both make a bolt in or weld in crossmember for your car. From the pics, they look like they would have more clearance on them. Chassis Engineering makes a nice bolt on side mount kit. Check out their website
www.chassisengineeringinc.com They have a lot of neat items for the earlier Chev chassis.
As for the length, I know there is something about a shorter water pump but I have no experience with GMC's.
