'69 Barracuda T56 swap, floor, cross member

I wonder how much the Holley G3 conversion mounts I am using will affect this since the motor is 1.75" forward of the LA motor location.

That would have a huge effect. The tunnel at the firewall is quite tapered, and you can see how the locations of the bellhousing bolts both to the engine and from the bell to the transmission end up in fairly tight locations with the stock tunnel. So, moving the whole assembly 1.75” forward should make a pretty big difference. I suspect you’ll still want to fold the pinch weld flat, but I doubt all the relief cuts would be necessary.