I experimented with modifying the governor on the Th400 to alter the shift points and have learned a great deal in the process. This article below explains the modification in the easiest language.

TH350/400 Governor Mods

Below are pics of the governor, and the inner and outer weights.

My first modification was the grind a small amount off both the inner and outer weights-I didn't weigh the stock weights but I removed what I thought was a small/conservative amount, just took the corners off both weights and left stock spring. This resulted in the outer weight being 41 grams for outer weight and 8 grams on inner weight. This resulted in very high part throttle shifts and WOT 1-2 shifts at nearly 7000 RPM. no bueno.

So, modified another governor, getting outer weight to 42 grams (+ 1 gram) and inner weight to 9 grams (+1 gram) and this resulted in decent feeling part-throttle shifts and WOT 1-2 shifts at 5500RPM.

So, adding one gram dropped the WOT shift 1500 RPM.

The scale I used only measured in 1 gram increments and so I ordered a more precise scale and am going to grind some more weight off inner weight (full throttle shift) to see if I can raise it.

FYI-getting to the governor was a bitch, I removed the transmission mount and then removed the weights from the governor while it was in the car, I cut the pins off with a dremel tool and replaced with 6D finish nails. I did this cause the governor can't be removed with transmission in car.

 

B&M make a kit with different springs, weights, pins and gaskets. $75 at Summit racing. I used the kit.
Part number B&M 20248
Oyher good mod's are a shift kit and adjustable vacuum modulator.