Small block headers: worst to best fitting. List your experience.

Dougs 1 5/8 on my 68 Fastback with A/T , power steering and breaks .

TTI 1 7/8 on my 69 340 4 sp Swinger with manuel brakes and steering .

Both fit great !
If you drop the body on the engine/trany / k-member assembly like the factory they all fit better .
And try truck isolators in some scenarios. They are thicker