well, there doesnt seem to be a whole lot you can do for these motors without serious mods.
as ive been searching the best thing to do is just dish out $2800 for a supercharger. but, if a large sum of money isnt on hand at the moment.
not particularly in order, but close..
Since you already have the filter, i guess it can stay (i have a drop in also) but a cold air intake (CAI) is better and will produce more power.
You could build your own "half assed" CAI using your stock airbox and some dryer hose or PCV pipe and it will sort of split the differance between the drop in replacment and all out aftermarket CAI.
next, well, u already have exaust, so thats good, but did you just replace the stock muffler?
I went to a 3" exaust, changed my muffler and had 3" bent after the muffler, but kept stock muffler to engine.
best thing to do is keep the muffler but have 2.5-3" pipe (Single) or 2.25-2.5" (duel) pipes bent. first, run a decent set of headers (the ones that come with a good Y or H pipe *the pipe from the headers collector to the catalytic converter*)
Then run a good aftermarket high flow catalytic converter. (or two if u have duels) thats why i chose single exaust, only need one cat and less pipe, also dont have to run the pipe near my gas tank or get rid of my spare tire mount.
So now it can get rid of the exaust gasses, the engine will run cooler (even if the temp gauge doesnt show it, your exaust valves will run cooler and so will the chambers). so we can build HP.
now we can cram even more air in there.
might as well dish out for a fastman TB, its worth the $200 and is a SUPER easy bolt in, no brainer, just remember, some idiot on an assembly line built it, u can take it apart easy!
Now you are making decent power with the engine chrylser gave u to work with.
Since you pull a trailer, i dont suggest the mopar PCM, but a friend of mine i work with does have the mopar performance PCM (the computer) and he pulls a 28' travel trailer all the time without engine pinging.
So, thats more of a call on your part, if you're willing to run 92-93 octane fuel, then go ahead and get the PCM.
You can make more power by adding the M-1 intake, but since ur buddy is using it for towing, you want your bottom end torque, so i'd stick with the stock intake for now but would later down the road consider upgrading to a ported stock style intake.
Next i would address the tuneup of the engine, purchase a set of NGK spark plugs, i got the power groove ones, very nice plug. this will help ensure all ur fuel gets burned.
also upgrade your distributer cap and rotor with brass terminal units. I think Napa or most any local autoparts store has them or can get them, make sure its solid brass, good quality.
Getting a set of good aftermarket plug wires will also help since stock chrysler ones tend to die young and wear easily.
You can also help keep the motor alive by changing the engines oil to Mobil-1 synthetic or even better but more $$ Amsoil synthetic. this will allow the motor to run cooler, less friction (free hp), keep deposits down and will generally live 4 times or more longer than with conventional oils.
also running synthetics in the rest of your drivetrain (transmission, front axle, rear axle, etc..) will free up HP through friction loss and keep the parts working for a long time.
now, if you still need more HP, and dont we all
You can add more lift to your stock cam by adding larger rocker arms than stock or better yet getting a new/recut aftermarket roller cam made to match your current modifications and even keeping in mind future ones.
You can also tune you electronics to perform at their peaks, like tuning your TPS (throttle positioning sensor), AIT (air intake temperature sensor), TB (throttle body) plates to ensure they open all the way, etc... but this generally takes more than just "bolt on" skill and some added tools like a multimeter or volt/ohm meter.
still want more power?
heck, change your heads and get a cam to match or just drop in a stock RT cam.
you can get the RT heads or ported RT heads with 2.02" valves and then get a matching cam.
even further you can stroke the motor or buy a drop in replacement 400ci stroker.
but thats $$$$ stuff.
oh, you can also upgrade your injectors to better performing (better spray pattern) or larger GPH or LPH (gallon or liter per hour) rating to increase the amount of fuel.
ummm... hmm.. theres more but ive been up for about 30 hours now
Hope some of this helps or make sense at least.
Goodluck, im headin down the same path myself.