A Muncie which shifts hard is going to have one of three problems
1) Wrong lube
2) Bad synchros
3) Faulty or improperly adjusted clutch
As the problem did not exist before the lube change, focusing on lubricant is, clearly, the first choice.
The reason the 75W-90 seemed too thin is its a multi-viscosity lubricant, ie: when the lube is cold, it flows like a 75W gear lube but when it gets hot, it flows like a 90 gear lube.
It is difficult to find SAE 90 rated GL4. This is because lubricant makers don't worry much about old cars and, today, want to sell GL5 gear lubricants which can be used in manual transmissions.
The problem is that GL5-rated lubes are designed for hypoid rear axle gear sets. With hypoid gears, not only do the gear teeth "mesh" as the gears gurn but the gear teeth slide sideways across each other, as well. This requires much greater propotions of extreme pressure additives in the lube than does a lube for spur- or helical-gear transmissions.
The problem with all those EP additives is they defeat the action of cone synchronizers in synchromesh transmissions.
It is possible your trans has this problem.
You're going to have to look hard to find a GL4 rated SAE 90 or 75W90 gear lube.
Many people don't want to spend the money for a high-end, ester-based synthetic for the four-speed in a driver, but Red Line Synthetic Oil Corp. makes a GL 4-rated, 75W90 gear lubricant. It's called "MT90". Back in the day, when my 71 had a Muncie in it, that is what I used and it worked quite well. Also, when I changed to a Richmond six-speed I also have used MT90 in that.
