Doug’s Headers on 64 A body 4 speed

Professor Fate said
Okay, you used new mounts.
BUT- which ones did you use?
These are listed for the early As in most all parts listings now (I just used RockAuto as an example, but NAPA/O'Really/Autozone etc. are the same...); but that number is totally, completely, and unequivicably WRONG!
2266 Series biscuit:
View attachment 1716068034
This is the correct 2338 series biscuit:
View attachment 1716068035
Note the difference!
Those headers are extremely tight in the engine compartment, and any deviation in the height of the engine mounting from stock will result in clearance issues such as you described, especially on the driver's side. The passenger side has a bit more room to work with, and may not interfere as badly.
So, if you used the 2266 mounts, swap them out for the 2338.
If you can't find the 2338s anywhere, use 2710 mounts (ex.: '91 Dodge D250 Cummins) which is the same thickness and has the added benefit of built-in torque limiters (negating the need for a torque strap or chain).
2710 series:
View attachment 1716068036

I agree 100%, but will just note that with the correct driver's side mount, two tubes of the driver's side TTI header in my 65 Barracuda touched the torsion bar, where the bar snaked in between them. So I switched to the later mount for that side only, which dropped the motor just enough for the torsion bar to clear. It's still close, and I thought it might still hit when the motor torqued over under power, but apparently it doesn't.

It's all very tight in an early A body, and there's enough variance in production clearances, that what works for one car might not for another, even with the same brand headers.