If you have a GMC Truck, the motor/mounts are for a V6 or I6. The GMC frame is different than a Chevy and the 2WD is different than a 4X4.
If I read your post correctly, you have a 65 GMC 4X4 and have access to a 63 Chevy 2WD, so examine the differences. Anything is possible and it's all been done by somebody. All you need is a tape measure, a plan, a welder, some fab skills and time. It would be a miracle if someone here knows the exact differences between those two trucks.
Check 4X4 frame channel is deeper/stronger, frame horns/mounting features different for bumpers/radiator/grille, Some GMC's are longer for long motor, motor and tranny mounts different (depending on what motor and trans we are talking), body mounts, coil front/trailing arm rear on 2WD, all leaf on 4X4, Xfer case mounting features, shock mounts, Hydraulic clutch vs. mechanical, and etc.
As you can see, you have:
1. Year differences.
2. GMC/Chevy differences.
3. 2WD/4WD differences
all at once.
Possible bag of worms, fab nightmare and Frankentruck looks. A lot does depend on your skills or your money.
Bottom line: Not even close to a "bolt on" swap.
