you will lose milage, i did on a 5.3 long ago
plus the loss of torque IS noticeable and it sounds like crap

 

One more thing,
The Gibson shorties that you have are pretty much identical to stock manifolds as well. I'm not bashing you at all. Just a little FYI.
Back to the subject line though... Like others have stated. The sound WILL get annoying. If you want LOUD, maybe get a cut-out. Otherwise just get a straight thru design muffler. To answer your question, it may or may not effect gas mileage (shouldnt be much either way). Whatever you decide, exhaust or not, I'd suggest a tune!
There is a lot to learn on this forum.

 

any exhaust shop should be more than willing to cut the stock muffler off and weld an aftermarket muffler on. just go online and buy a Single in Single out muffler and take it to a shop to be welded in. Or cut the stock muffler off and use Band Clamps to hold the new muffler in place.

 

Would the band clamps allow the muffler to seal properly? The factory exhaust is 2.75, and most aftermarket mufflers come in .5" increments, such as 2.5, 3.0, etc. When I bought my 3" cutout dump pipe and 3" Magnaflow muffler, I was test fitting them using band clamps and couldn't get them to seal and tighten up at all.

 

very good observation. seals fine as long as both are same size and you have the proper clamp.
Tried finding a 2.5-2.75 clamp but didn't see one. i say just have a shop weld it in place. shouldn't be much.

 

Local shops around here will weld these up for less than $50.

 

My exhaust guy will weld up a new muffler and dump tip for me for $20 bucks, me providing the muffler of course. I would think that would be the way to go.

If nothing else, Dynomax race bullets are what$30-40 bucks?