If you cant afford to rebuild the engine you cant afford to get the " most power " , but there are some things you can do to get some more power. I wont go into the high end stuff because your talking some pretty big bucks, but yes gears are the best step and on a budget will give you the most bang for your buck. 3.55 3.73 4.10 and the list is much longer than that but the most common are 3.73 for manual trans and 4.10 for the auto.
Gt40 stuff is great if you can find used from an exploder, and a good B cam . People that have done the triple head, intake, cam have reported a 30 to 60 hp gain depending on exhaust but I would tend to believe closer to the 30 or 40 hp range. Get rid of the stock headers as they are far to restrictive , any thing you can do to the exhaust to open it up will compliment that combo. Remove cats if you can long tubes with O/R H or X pipe 2 1/2 inch all the way into some good flowmasters , but I see your in cali so you will have to keep in mind your smog tests out there. Removing cats is probly not an option for you .
I promise you if you spend the money for some ford racing 3.73 or 4.10 gears that engine will come to life . I wouldn't bother with pulleys because you really only free up a few hp and they can cause some cooling/charging issues at idle . Some pple swear by under drive but Im not one of them. I do have a set and idle charging is ridiculously low for me.
Gears are best left to a reputable shop that knows the ins and outs of the rear differential , but look to spend somewhere around $400 bucks for gears and professional install . That of course is an estimate because shop labor varies location to location. If you are confident you can do the install then your looking at about $200 for the gears and install kit. I am thinking in 91 the rear axle was 8.8 with stock 2.73 gears , and there were optios for 3.08 in a manual car or 3.27 in the autos.
Before running out and ordering a set of gears and deciding to install them your self read this pdf tech sheet from ford racing on the install procedure. Read it until you completely understand every step of this as a botched gear install will not go well.
This tech sheet is usually enough to keep inexperienced mechanics from even attempting this on there own.