I used Rhoads lifters in my stock '66 Merc Monterey w/410cid V8 4bbl. Mpg went from 19hwy to 27hwy. Vacuume from 16in to 22in. Very happy with results.
Have you considered decreasing rotational mass? Lighter pully system, rims, driveshaft, etc. also brake springs to keep pad pressure off disk.
Belly pan: Online pic of underside of car spray foamed then fiberglass covered. You would have to be REAL confident on rust proofing. Driver did say the lack of air turbulence and foam made for a remarkably quiet ride.
Last word of advice: These projects can cause a lot of opportunity to overthink. Their will always be a "maybe this is better".
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