I did some research on this a while back, and it seemed like every thread I found talked about drilling holes in the firewall or other modifications that made this swap difficult to do.
In looking at donor trucks, it seems like only the metric (1980+) trucks had the type of booster I'm working with here.
Anyways, some pictures:
Comparison between an unmodified '87 hydroboost bracket and my '72 power brake booster
Lining up the center holes puts the pushrod in the center of the hole in the firewall. I should note that the hydroboost had two pressed-in studs in the center holes, which I popped out.
Using two snug fitting bolts and a welding clamp, I carefully lined up the two holes and marked for the other holes I needed. It's tough to see here, bu there was a stamped bend in the top of the hydroboost, which I pounded flat prior to marking (just below the top holes)
After drilling three new holes and some cutting and grinding, this is what I came up with. The cut on the bottom right was to allow me to bend the bracket out slightly to clear the flange around the steering column on the firewall.
Bracket dry fit on my '72 cab
And here's the donor hydroboost bolted in place
I had to make zero modifications to the firewall, except pulling out the original rubber linkage boot.
The photo doesn't show, but mounted like this the hydroboost pushrod is about 5/8" too short (and offers no adjustment). Unfortunately it's a smaller diameter than the adjuster end on the 67-72 style pushrod, so either I will cut and thread the rod and thread a piece of tubing to make an adjuster of my own, or I'll modify the pedal to move the pivot closer to the firewall.