I would go with a crower cam . Level 2 or level 3 installed 4 advances. I would inspect the pickup passage of block. I hope that cover replacement solves the problem.
Yep, a Crower level 2 or 3 or a TA 112, just above stock and about the same as the Comp 260. The TA and Crower are dual pattern cams which would be better for the Buick motor as compared to the Comp Cam 260 series which is a single pattern cam. I had real good luck with the Comp Pro Magnum Lifters.
Do NOT use RTV on the timing cover gasket. Install it dry, or at the very most, use High Tack spray-a-gasket. RTV is WAAAAAAY overused on gaskets
