I'm honestly not sure what all would be needed to swap to power brakes, but I'm sure someone here has done it. Probably pull from another year G10, but I wouldn't have the slightest as to which years. I doubt a car or truck booster would work as a direct fit because of the difference in firewall angles, but you could probably modify one to work if you're talented with that sort of stuff. If you find something in a scrap yard, don't forget to grab the proportioning valve.
As for the gas tank, I've searched all over the net and don't see any for a 73, everything is for 1st GEN or 77+. You might be able to easily modify a 77+, but then you'd get into a different sending unit. White vinegar is my go-to rust remover, but there are all sorts of methods for de-rusting tanks out there. Good luck.