I truly hope this is not another "fad away" new guy. Hope he stays engaged and checks the post. Some just don't realize that Stovebolt has a massive amount of knowledge that is put forth within minutes. He has 8 same-day responses. Several second day.
1. Does bucking mean the truck is lurching or hesitating or bogging down during driving? Is it rapid or large intervals?
2. Does it mean the motor itself is jumping all around while truck is not moving?
3. Does it do it only while going thru gears using clutch and then settles?
4. If you get up to 30 mph and push clutch in, does it coast with no bucking?.....while motor is revved a little?
5. Do motor mounts look OK? That motor is pushing 900 lbs on two rubber blocks.
6. Was it running good and all of a sudden did this? Was some work done just before this? I realize you did work after.
7. Can you describe bucking....what, when, where, how long. Trying to determine if "bucking" is the motor missing.... or the truck (motor mounts, clutch, tires, brakes, ujoints, wheel bearings and all that stuff). That would really narrow down "bucking".
8. Could you find a place to safely coast with motor off, clutch in?
9. Any modifications like mustang front clip, air bags, lowered, lifted, big tires, wheels, messing with distributor.
10. Those motors run better at more advance than spec now-a-days. Like 8-10 BTDC sometimes. Spec is 5.
11. If you have removed spark plug wires, recheck that you have them clocked for Firing Order 1-6-5-4-3-2 NOT 1-5-3-6-2-4
12. "When a pump goes, it goes". You can't use that theory to eliminate the pump.
Another issue OldGMC1963 is we don't know your mechanical/electrical automotive experience, equipment or ability to test. Sounds like you are no rookie from all the tune up effort you've done. Let us know your approximate experience level. Sounds like a neat truck. I had a 1960 GMC with 305E recently. Yours is first year for coil front suspension and last year for wrap-a-round windshield.