Even back in the day when I maintained a fleet of vehicles including 50s and 60s GMCs, we were single-sourced on many parts and had to go to GMC and pay $. Macks were similarly pricey. The situation is not better 50 years on. Originality has its price and you might consider another solution that fits in your engine bay; perhaps a 63 or later Chev I6. You do have the motor mount issue to deal with but that can be addressed with the right choice of bell housing. I would avoid cutting out the cross member the back of the stock engine sits on if at all possible. It's a structural part of the frame and sits directly under the cab mounts.
In 1960,IIRC GMC replaced the 228-302 engine with a 305 and 351 V6 and even a 702 V12 all based on the same block. These were heavy, slow-turning torque monsters that sucked gas like there was no tomorrow and were horrendously expensive to overhaul. Vast numbers of them were pulled in favor of 327s and the like, even when running just fine. So, you don't have a late-model GMC option.
