55+ vibration - Help

[tdangle]

OK, I have searched and found lots of posts about vibration, most either fixed by something obvious or no solution found.

I have a C20 leaf springs dana 60 2 wheel drive long bed. GM performance crate motor (SP 350/357) with a newly built 7004R from Bow Tie Overdrives. 2 Piece drive shaft rebuilt, shortened and balanced by a drive line shop. 265/75R16 tires with aftermarket rims. New tires road force balanced. Operating u-joint angles .2,1.9,3.7 per spicer app calculations. Tire pressure 40 front, 45 back. New disc brakes up front, new drums in back. New front end rebuilt and alignment.

Vibration is same in 3rd or OD or neutral. Accelerating or decelerating no change.

Very smooth below 50 MPH. Braking is very smooth with no shutter. Mainly feel it in my stock bench seat, do not feel it in my feet on the floor, center rear view mirror shakes

Anyone have ideas