1980 Chevrolet Silverado smoke smoke and more smoke

My dad had bought a motor that was rebuilt from a willow Missouri company called sequel I still have the tag from the motor he put a new cam in the engine the correct way and it set a year and then we cleaned and painted it and it sat for a month and then we got it going and now it smokes like no other. It's a 1980 350 that I just moved into a different truck and it now smoked where as it didn't in the other truck and my dad and I think it is the ring seals and wondered what else it could be and if it is the ring seals could just adding a ring seal repair additive to my oil solve my problems? It burns about a quart of oil every 20 miles but only smokes when the RPM's are up around 3-4. We added some Lucas stop leak and have used synthetic oil to slow down the burning but it wont stop. It burns about a quart of oil every 20 miles.