I did the 3.9 to 5.9 swap on a very tight budget so i didnt replace the trans (42re=3.9...46re=5.2) the trans slipped and acted bad with the v6 in the truck but when i did the swap i couldnt afford the v8 trans to put in my truck.. So i changed the fluid and filter and used type f fluid...it has a friction modifier in and has actually prolonged the life of my trans.. I am not sayin this will work in every trans but its a cheap (try) to fix.... As of now the trans still pulls and hasnt let me down yet...i will replace it with a 46 re this summer tho!

 

Cool! I'm glad its holding up.