You can cut the effect of the 4.10 axle by going to larger tires. A 33" tire should fit on your truck without any mods. Something like a 285/70/17 or 285/75/16 would work. A very standard truck size is 235/85/16, but its a little shorter (32"). The cheapest choice is usually to use 16" wheels, since they were offered stock on many trucks, but the stock GM wheels are only 6" wide which is is really too narrow for anything more than about 255 or 265 width tires.
You could swap an Edelbrock 1406 600 cfm carb and maybe pick up a little mileage. You would need to tune it for your cam, since the stock jetting is probably too lean. The stock step-up springs will also create an incredible bog with a high lift cam because it will transition from cruise to power mode way too late.
I have a very mild 350/TH350 in my '75 GMC truck, and I can't get much more than about 10 mpg if I drive it on the highway at about 55-60. If I kick the speed up to 70, my 4.10 gears drop the gas mileage down even further. These trucks are built like a brick, and its hard to overcome the drag. I'm running stock height 31" tires, but I haven't wanted to spend money on trying any other combination.
The swap to a TH700R4 is fairly easy and would help highway mileage, but you will have to spend money on building a good one if you plan to still race it.