More Power/ Thumping noise

I have a 93 chev 1500 it has a 305 in it and it is gutless. i was wondering if there is a chip out there that helps or soemthing because i am trying to stay with the smalleer engine instead of the 350 cause the gas milage is better but going up hills with the 305 is like going backwards. lol. also when i turn the truck on something under the dash thumps for about 30 seconds and it does it when i turn the truck off for about the same time. I believe it is the recycle part of the air conditioning because if i press the max air when it is doing the noise it will stop for a bit then start again. any help is apreceated.
thanks
Chris

your truck has a special ecm that cannot be modified using an aftermarket prom because it has a memcal type prom that contains the knock sensor, making performance chips unavailable for your application. A few tbi, intake and exhaust tips, and freeing up the air intake side, like an aftermarket free flowing air intake are about your only options, other than giving up a MPG or two and swapping in a 350

In most cases the 350 will get better gas milage than the 305. My truck is an extended cab, with a 350 and automatic, it get between 17 and 18 MPG, city and hwy combined, and I have a heavy foot.