Welcome to CHR! Pat McCarthy is our resident BB expert - none finer - if it's been done, he's done it more often and better than anyone. He will be able to give some good advice about most anything aftermarket or internal for big blocks.

With my "limited" experience I'd recommend that you first make sure that any modifications you make will stand up to emissions laws. Washington State requires emissions testing every 2 years and I believe Spokane County is included.

GM has done a pretty good job of compromise with mileage/performance and "noise" on the big dogs. What you can do is performance intake, such as K&N (7 horsepower) and a cat-back exhaust from Gibson (5-10 horsepower). Both companies meet CARB requirements which Washington recognizes as emissions compliant. The intake in particular will give a noticeable bump as it removes the stock air filter housing and you're no longer sucking air through a tiny little funnel.

The throttle body and MAF sensor are two more components that may have a few horsepower hiding in each of them but make sure the throttle body is compatible with your cruise control. Key for the MAF is overall air flow. Granatelli make a high quality replacement that flows 60% more over stock that may add 8-15 HP.

So - with "bolt-on" components that won't violate your emissions laws you can get 20-45 HP or so.

You may also be able to tweak your computer programming to raise the shift points allowing you to better utilize some higher RPM when towing.

Good luck and once again, welcome to CHR,
Glenn