How much RPM drop from park to drive with stock type 340 convertor

Put it in drive, putt along about 10 mph and stab the gas, watch where the tach needle jumps up to.
That is the flash on the convertor. At the end of the day, that is all that matters.
Where the car wants to creep past where the brakes will hold it is entirely case by case. How good are brakes, loose is convertor, size of tire,etc... Hard to draw much conclusion on what the convertor is based on that.
I have had 5k convertors i couldn't hold without creep at 3k....sounds like what you have wont be much different than what a stock one will do.