I'm part of the pit crew for a friend who vintage-races a 1965 V8 Sunbeam Tiger. It had originally, a similar pair of struts between frame and firewall. Another crew member, who is an excellent welder, added an angle steel brace across the firewall to reinforce this area. He uses a Heli-arc welder.
Get the best welder you can find for an opinion on your problem. This wlll be difficult to solve via E-mail.
The kit I designed for my steering problem mounted to the outside of the frame rail so there were no 'ridges' to displace the steering gear. I put additional plates inside the hollow space to further reduce flexing.
The whole arrangement is now bolted in place with 7 bolts instead of just the original 3.