I have a 1978 F-150 with a 400 and C6 Transmission. I rebuilt the engine about 12000 miles ago (Fall of 1999). The engine is great. I used a bunch of information from this website to build a real stump-puller. At the time, I replaced the transmission with a rebuilt unit that was supposed to be set up for towing. I use my truck to tow a boat almost all the time. In fact it is always hooked up to my work boat. I would guess that at least 9000 of those miles were towing either my boat or travel trailer. I use other vehicles to do any running around. It has an additional B&M cooler which is hooked up inline with the cooler in the oversize radiator. I have noticed the last few trips when I was hauling my boat that the transmission seems to be be slipping on its shift from 2 - 3. It seems to shift kind of slowly and allow the engine to build up some extra RPM before it makes the shift. Sometimes it helps if I let up off the gas for a moment. It generally makes this crazy long shift when I am going up a slight grade and am accelerating to about 45mph. I have not noticed any problems with it shifting into any other gears including reverse. I checked the fluid level. I disconnected the vacuum line that goes to the vacuum modulator on the transmission and sucked a vacuum on it with my vacuum pump and it holds. I am going to contact the rebuilder tommorow but was hoping to hear some more words of wisdom from the group. If you have any additional checks, please let me know.

Thanks
MasterMarine