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The most accurate way to get tire revs per mile is to use a cloth tape measure, used for sewing, to measure the new, mounted, inflated, unloaded circumference of the drive tires, use Pi to figure the diameter, then use 97% of that in all your calculations.
20168.0672268 ÷ diameter, then x .97, will also get the tire revs / mile.
Gm speedometer calibration is also super easy for the 700R4, and cheap, too. More accurate than GPS, by a lot. You need to know your axle ratio, then you can try it with all 4 of the available speedometer drive gears, to find the closest match. But then there are speedometer correction gearboxes out there, small enough to fit in your closed fist, to get it dead-nuts perfect.
I know the Caprice is boxier than a 78 Camaro with the 5" rear spoiler, but drag should be similar, because of that spoiler. One of my 78s had a 350, a 700r4 i swapped, a 3.08:1 axle, 235/60r15s, and returned 25.1 mpg at a steady 65 mph, including some hills.
It was one of the best combos i ever drove, and i am very strongly tempted to replicate it in my current i6 Camaro. I fe e l some tweaking could get it even higher, both in mpg and in performance, partly because i later discovered that my distributor's centrifugal advance hadn't been working.
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