I whould like to go to a newer & stronger transmission for towing pulling.

Check the trans fluid level (the truck needs to be running for this) if low then you're lucky just add more fluid until the level is within the limits on the dip-stick, do not overfill. If the fluid is up smell the dip-stick if it has a strong smell (burnt) you will need a rebuild. Start there.

AGREE WITH #1 BUT YOUR STUCK WITH THAT TRANS,BUT IT CAN BE UPDATED