How to determine the needed stall speed?

UOP said

Personally, I wouldn't go any lower. Heck, I'm thinking of pulling mine and having it loosened to 4000 over the winter. If you've already got a 3000 stall (what size btw, 11"?) then rock it. I don't know if anybody has mentioned it yet, but ditch the radiator cooler in the radiator and get a good aftermarket unit, as big as you can fit/afford. You can't have one too big.

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well this where i fit the stereotype mentioned in this thread several times. I cheaped out getting my magnum swap together.

The converter is a low mileage PTC i got from a member on here for 200 bucks. I'm not sure on the size, but the member stated it "flashed to 3k"(looking back at the PM), so it may be lower than a 3k stall, which now has me worried that maybe my transmission is in way worse condition than i thought. If the converter isnt a 3k stall then my 3rd gear is badly slipping on the highway. Because past threads& discussion have felt that my highway rpm's at 65mph were about on target for 3.55 gears and a 3k stall converter.