Hi Red'.....
The transmission is a weak link, no doubt about it. But you know... not much has changed really...
Right from the very start, whether stick or auto, they seem to be able to take perhaps another 60-70 rear wheel HP over stock as the upper limits of long term reliability.
Anyway, the stall on our TC's is (according to the book) 1700-1900 RPM.
The higher you push your HP the higher the stall will get, until finally you just push right thru it and roast the little booger...
If you have the bucks, the aftermarket full meal deal is THE way to go. All I can say (to be brief) is I figure I was loosing 30% of my $$$ and my HP upgrades between the engine and the rear wheels...
The difference is truly day and night.
Now, you can push the ole girl up to about 225-240 RWHP and still have some measure of reliabilty.
Have a good service done on it. Full check out,,, including line pressures, and adjustment if needed.
There is an "acceptable range" for your trans. 'idle line pressure' and your wide open line pressure.
Have your shop set both at roughly 75%-80% of that max value.
This will help reduce the likelyhood of pushing a seal out or damaging one.
Next, use a more aggressive ATF. Try Dexron type II.
It has less friction modifiers in it and will help to firm up your shifts.
If you want to go synthetic then I'd suggest a semi-synthetic. They are very "slippery" oils and result in more slippage on each shift and while in each gear... especially under higher HP levels.
The tip about the TPS is a good one. We've got some pretty detailed stuff in here on that topic. Do a search and make yourself familiar with it.
Since your truck is gettting on in years, it can help on initial startup to put the trans in "N" for the initial warm up too.
This will allow more oil flow to charge the trans lines/valve body/TC etc, even though our transmission's apparently do pump oil while in "P".
Well, that's about as confused as I can make it for now... .
Pastor Bob... .