First, did you polarize the system when you changed the voltage regulator?
The positive ground GMCs of that era had a very comprehensive section in the maintenance manual on voltage regulator testing and adjustment. You should look for a Chevy maintenance manual covering your truck and use your "ordinary multimeter" to test and adjust. Just be sure not to take any current measurements that exceed the ratings of your meter, because it is real hard to get the smoke back in once you let it out.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
| 
