i want to get 35MPG out of my pick up truck

[theholycow]

Those are some good thoughts, and it's a great idea to remember to try additional strategies. In my observation, exhaust and minor weight loss won't make a difference.

Weight: That truck weighs at least 3200 pounds. At most, the spare and jack could weigh 80 pounds (and I doubt it's that much). With a few exceptions for specific models, I've observed that nobody has reported a credible FE change from adding or removing less than 10% of weight, and trucks usually seem to require even more. Also, it sounds like the OP is doing highway driving; for highway driving, weight has almost zero effect, which is why some very heavy cars have very good highway FE. If you get it for free, I doubt it will hurt anything, at least.

Exhaust: People trying to save fuel tend not to use even a small portion of a stock exhaust's flow capacity. That's doubly true for highway driving. There are some decent arguments to the contrary in the exhaust link in my sig, though. Either way, it's almost assured that an aftermarket exhaust will never pay for itself.

The additional aero mods are definitely a good idea. Some people have success with underdrive pulleys and fuel additives but I'm not sure if they could ever pay for themselves.