Did you check the motor mounts for separation. They get old and the rubber separates from the steel or people use the oil pan to jack up the front of the car and tear the rubber. so they can be tight but not connected, if that makes sense. I've had that and the engine torqued, rotated up a little and the fan cut my upper radiator hose.
chevy runs deep, Don't step in chevy!
67 Coupe 289 - TCI C4 w/2k stall; Holley 4150 570cfm; Performer intake; mild cam; Pertronix HEI; 302 heads; Roller Tips; Hedman Tri-Y headers; H-pipe w/Flowmaster 40's; 9 in posi 3.50; 205/50-17F; 225/50-17R.