on the firewall, you could get the floor done for strength, minus the welding to the section of firewall that is where the booster etc goes, then mock up pedals and column locations and cut a section out and replace the whole section with a new piece. all the holes would be gone in that section. fab the part that has the booster and column cutaways in the right places for your needs. support the back side with bracing to support the pedals, booster etc. you would be all in with one shot. same on the other side. there are lots of holes to patch so you would be quicker to replace sections with nice new cold rolled flat stock. like 16ga that welds up easy. if you don't mind the look you could even do a flanged panel that bolts on like the parts used in trailblazer/envoy SUV's .
check this thread out. post 100. cool idea. post 246 shows the flanged panel that goes on from the inside and mounts the master cyl and booster as well as the pedals and drive by wire accel pedal on the inside. there is also a spot for the steering column to go through with a big grommet to seal it off. these have a column with the u joint under the dash instead of under the hood, may make sense to get the column where you need it for the engine. the further up the u joint is the easier to line stuff up sometimes. post 286-287 shows it as well. 301 shows another truck using the idea. note the one guy is using the trailblazer/envoy column with an old style steering wheel. kinda cool. may not be your thing but it's an interesting thread to go through if you have time. lots of home engineering going on that could spawn ideas for your own build.
https://67-72chevytrucks.com/vboard/...=348244&page=4