Bolt ons will help you get a few extra hp, but I wouldn't expect more than 15-30 with your engine depending on the modifications.
An aftermarket 'intake" that replaces your stock plastic tubing, airbox, and panel filter with a 3" or larger straight pipe and conical cleanable air filter, should help allow your engine to get more air, and cooler air, and make a little more power. This is a simple job your average mechanic can do himself.
a "cat back" exhaust replaces your restrictive factory exhaust and mufflers after the catalytic convertors. It will give you a nice growl under throttle and again, free up your engine's ability to expel waste gas. This is a little harder to put on, and your truck will be louder and may drone on the highway. It is also more expensive (will cost at least 200 for a decent one, more for options like stainless or polished mufflers and tips, or dual exit tips). It would probably free up some power even over your dual flowmasters, because the systems come dyno tested from the manufacturer. If you just buy mufflers and let a muffler shop weld them on you don't really know what you are getting (and what restrictive parts of the factory exhaust they are reusing, or what elbow they have crush bent down to half their original diameter, hurting your performnace).
You could also look at getting a "chip" or programmer to allow your truck to run an advanced timing curve, higher rev cut, eliminate speed cut, etc. This can give you some more power, but an aggressive program to get the most power out of your truck will probably also require you to use premium gas. On a gasoline engine these tuners usually are only good for five to possibly ten hp. (they really shine on turbo diesel motors).
Finally you could get a set of aftermarket tubular headers. Options range from bolt on replacements for factory headers to long tube race type headers that eliminate catalytic converters (which is illegal for road use and won't pass inspection if your area does smog checks). Headers will usually give a 350 an increase of 10 hp for factory replacements to 20 or more for long tubes. If you did all four of the modifications above you would have a weekend's worth and probably over a thousand dollars invested in your truck, to gain maybe 30 hp or so altogether. Keep in mind if your area does smog checks you will need to ensure that all your parts are smog legal (a carb certified sticker for california emissions usually means parts will pass emissions in any other state as well).
If you just want a fast and cheap hit of acceleration you could try installing "nos" or nitrous oxide. Expect to spend $500 for a new kit though you could find one less used. You will have to use premium gas and you'll have to get the bottle filled. It only lasts a short time and can risk blowing your motor if you get too bottle hungry. I woulnd't recommend it for more power for towing or daily driving. The bottle runs out too soon and you run too great a risk of ruining your motor if your fuel mixture leans out while under boost.
You didn't say what transmission you have, but if you have an auto you could get faster acceleration if you install a shift kit that allows higher rpm's and faster shifts, but it will hurt the "driveability" of your truck because of the firmer shifts. You could also try a higher rpm stall converter to allow you to build up more rpm's (and usually more power) before you launch. This will also affect your street driveability if you with one that has too high of a stall. This modification is usually most useful if you install a cam to shift your power band higher up in the RPM range. A cam might be something you want to think about, as it would make the most of the intake, exhaust, and headers mentioned above. Its also more complicated to install though, and should be left to someone who knows what they are doing.
Finally, one thing anyone with a RWD vehicle can do is switch out your rear ring and pinion for a lower (numerically higher) one. This will improve acceleration at the expense of top end, and it will increase your rpm's in each gear at a given speed, so the end result will be that you will see higher rpm's (and fewer mpg's) on the highway. BUT it will give you extra grunt in every gear for towing, and it will bring you off the line faster. You can find them in a junkyard if you know what axle you have, or you can purchase them through the aftermarket. If your truck is 4WD make sure you install the same ratio gears in your front axle at the same time, otherwise you won't be able to engage 4WD without damaging something.
I don't think you could shed much weight off a pickup truck without hurting its purpose, so while it would help your acceleration, its not a real useful suggestion in your case.