You have plenty of room. No need whatsoever to modify your header tube.
I have about 3/16" between my headers (H-O Racing Tri-Ys with 1-3/4" primary tubes) and the steering shaft on my '64 GTO. I tried to put the end of a 1/4" thick wooden yardstick between the shaft and the header tube and it wouldn't fit in there.
Exhaust heat will not adversely affect the steering shaft, the Ditzler 9000 black acrylic enamel on mine is still doing fine after 10 years in the staging lanes and 2 years on the street.
My GTO has stock rubber motor mounts and a chain to limit engine movememnt. Engine torque will make the engine move away from the steering shaft.
I'll be installing a new set of Energy Suspension poly motor mounts when I drop the LS3 in my '64 Tempest, they should do a good job of limiting engine movement all by themselves. Hopefully I'll have as much clearance as you do in that spot.
