Tom - Thank you for those calculations. God help Volkswagon's diesel engineering division if you ever decide to become an investigative reporter!
I dug up a historical reference from Wheels magazine, December, 1970 on an MG Midget they tested.
The quoted data gives us a maximum of 65 HP@6,000 RPM. How accurate the factory numbers are will remain conjecture, but the magazines observed data indicates a top speed of 86mph in 4th, which, with a 22.5 inch 145R-13 tire, put things at about 4,950 rpm.
Assuming a linear slope of horsepower on a stock 1971 Midget (certainly not a given), peak advertised torque of 72 lb/ft occuring at 3,000 rpm (41 hp), 4,950 RPM should equate to ~57 hp - which is what the magazine claims propelled the car to a top speed of 86 mph.
Provided we're making more than 57 hp at 85mph, I think we'll be good on that shift.
It's the 3-4 that's on the borderline.