I'd continue with friction reduction: tires, alignment, brakes, etc. The O2 is a better bet than a pyrometer, especially wideband. The Qjet deserves Doug Roe's book as one needs to understand how to adjust the part-throttle F/A ratio.
The HEI is a decent piece but I'd rather have an MSD Alll-In-One Ignition (with the #6 box) as you will want to
change the entire advance curve not just the initial advance. The HEI is not as receptive to this work as the MSD. (And chase down all the vehicle grounds now [bonds] as electrical reliability on an ancient vehicle can drive you nuts. Get a copy of the FSM).
Grille block, engine blanket are good to try. GM used the emissions controls of the day to maintain
constant 100-115F air at the carb intake. A Robertshaw/
Mr. Gasket "Balanced Sleeve" HD 195F thermostat, HD clutch fan (with full shrouding) and HD water pump also make the list.
Oldsmobile tried for highway FE with the
Turnpike Cruiser. Wish I'd found those articles 35-40 years ago. That CCC air cleaner system would be worth replicating for the Qjet. Highly.
You should state the use of the truck. Most gains, IMO, will be in planning the use thereof (combined trips, route planning, engine time versus miles, etc). A truck used for service throughout the workday is different than one driven here, then there. A guy commuting down from Nederland to Broomfield and back daily is a great deal different use than up to Eldora and some other sites on back roads and then back on down as a skilled worker. Mountains or the flats?
The low lift of the cam is okay to a point. Modern SBC cams -- roller -- feature short duration with very high lift (more area under the curve). Weigh the truck (empty, but driver + fuel; and loaded as in daily use), measure tire rolling height, double check gearing, record
vacuum gauge readings and post questions about the "ideal camshaft" for your truck/intended usage on a vehicle-specific board. You'll also want elapsed times from an
engine hour meter to see your overall average speed tank-to-tank.
Establish good record-keeping to find overall operating cost per mile.
Read up on how to
performance degree a camshaft if not already familiar. Some teeny adjustments at that stage are beneficial, (not just "straight up" or 2-degrees advanced, etc). And ask about best OEM cylinder heads so as to make any runs to Denver-area boneyards beneficial. Later heads are miles ahead of any 1970's pieces.
The right cam and heads will do a lot to maximize FE. Least fuel for most work being key. But one must work from verified numbers to achieve this. Anything written by Marlan Davis and David Vizard about FE is vital (mostly performance stuff but I spent 30-years reading hot rod mags to glean the tidbits of FE and heavy vehicle performance). An SBC can be put together many ways, and combinations of cranks, heads, cams, etc, can be mixed endlessly to find the best torque curve
for your intended use.
Warm-up from cold is the biggest impediment. Even here in Texas my old cars took about 30-miles of sustained driving before they were feeling loosened up (coolant, oil, greases, tires, etc). Anything to shorten that would be high on my list as anything shorter in miles is compromised (for an empty truck; heavily laden would be faster) in the quest for best operating FE.
How much time the truck actually spends above 50-mph will dictate the benefit of aero beyond a minor air dam, etc. Only records will show this.
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