Is the stock exhaust in good shape? If the converter is okay pull off the muffler and put on straight pipes out the back. Split the pipe after the converter into 2 1/4" duals, no muffler, and run them out the back of the truck. I had this set-up on my '89 and it sounded bad ass.
Cheap HP is not going to happen. A few things to do would be headers and a good ignition but you will spend upward of 4 or 500 to do either and you really won't see that much of an increase. I would suggest a Jacobs pro-Street Ignition if you HAVE to do anything. I saw a substantial increase in mileage on my '95 with that ignition.
Mark