My '79 C2500 is moving the same way. 454 manual SM465, with 3.21 rear end so already geared pretty high for highway. At 55mph reasonable RPM's in low 2K, at 85+ it nears 3K which is more than I like for distance cruising when doing the Western parts of USA.
Engine rebuild nearing completion, compression bump to ~9-1, full roller cam/rockers, cam is an Isky 112LSA 205/215 with near .500 valve lift. Mild build, mild cam, Tri-Y headers 2.5 duals. A torque monster build which the power should be strong from ~1600+ Heads were swapped from the peanut ports which die around 3500rpm for ovals which can flow to 5K+, not that I plan to run it at those RPMs. 1500-3000 is where I want usable torque. "Financial friendly" is relative, this build is looking at least 7K all said and done.
I am strongly considering adding a Ranger Torque splitter which fits between bellhousing and transmission and all slides back 7.5" with shortened driveshaft. Adds a 2nd gear shift in the cab so you can engage the .30 overdrive, each gear can be split (2, 2H, 3, 3H, 4, 4High... reverse, reverse High..) More gear choices to match power to speed, lower revs when cruising which could translate to 17mpg in my dreams. (from the current 11@85mph)
https://www.summitracing.com/parts/add-760002m-27/applications/engine-family/chevy-big-block-mark-iv
That link is for the big-block input shaft size needed for my application. $1600-ish for the part, +install, +welding for your front and rear drive shafts... easily adds up and hard to justify the price just on gas mileage gains alone. I want the gearing torque options for towing. I also plan on keeping this truck essentially forever because I like it. There are cheaper/smarter options out there with more modern equipment.
Tool for selecting gear ratios:
http://ctny.audiworld.com/mark/s4/gears/gear_calc.html gear, rpm, speed calculator
Because the truck aerodynamically is a brick, keep in mind power needs increase with the square of the speed, e.g. if it takes 70hp @55mph to move the truck, @80mph the extra power needed to overcome wind resistance increases rapidly to 120hp+ and more gas. If you can live with lower cruising speeds it helps gas mileage a lot. Things can be done to reduce the coefficient of drag a bit making it smoother, check this thread:
https://ecomodder.com/forum/showthread.php/practical-guide-pickup-truck-aeromods-32386.html
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I have some clear Lexan and intend to pursue the Wing+Cover option. A partial 60% tonneau cover helps a good bit. Apparently between the cab and the tailgate a rotating mass of air helps support airflow from the rear of the cab over the bed to tailgate. Not quite as smooth as a boat-tail Kammback design, but helpful to smooth the airflow. Add sideskirts, a bit of airdam to keep air out from the underside of the truck, maybe a belly pan... small things add up. The hard part is not going so far to ruin the look of the truck. A certain amount of lipstick on this pig I am willing to test, but it is still designed a pig.
