My trusty work truck (1991 F150 5.0, AOD, 3.55 non-ls) of 8 years is getting pretty tired. Now the transmission is slipping pretty frequently and the engine is pretty weak. I've put a lot of parts and labor into it over the years so I've decided to keep keep her on the road.

Since I'm going to replace the engine and transmission it sounds reasonable to choose the best combination to suit my needs. I do home remodels so I always have the bed loaded down right now. My plan is to get a utility trailer and keep all of my tools in that rather than the bed of the truck. That means I will be pulling a trailer most of the time. Should I go with the drivetrain I currently have or make changes? I won't be changing both at the same time. I'll replace the transmission first then the engine. Thanks in advance

 

Cheapest / easiest will be to stick with what you currently have. Anything else will be enough of a bigger project that you'd better be relatively unhappy with what you currently have to justify it.

That said, if you're doing it in stages you are a little more limited (which can be good, if the sky is the limit projects can get really big in a hurry). I can think of four "upgrade" possibilities for the trans:

• C6 auto - no overdrive or locking torque converter and it's known as a relatively inefficient trans, so it will hurt your gas mileage significantly, but it is stout.
• E4OD auto - needs an electronic controller. Usually this is the same computer as the engine, but adding it later you might want to go with a stand-alone controller. Of course that runs the cost higher, but it's a strong, efficient auto trans.
• ZF5 manual - might be the strongest and most efficient option, but it means switching to a manual trans (whether that's a good or bad thing is up to you)
• built AOD - not sure exactly what you'd want done to it, but AODs can be vuilt for more strength than stock.

Then later for an engine you'll need one that attaches to the same trans as your current 302:

• 300 six - definitely less power than a 302, some say better usable torque and better mileage (while others say the opposite)
• 351W - definitely more power and torque than a 302, some say significantly worse mileage, others say not much difference

And of course any of the three engines can be built too.

If it were me I'd go with the ZF5 (because I hate automatic transmissions) and probably a mildly built 351W. If I wanted to stay with an auto I'd probably keep the AOD and maybe see what it took to get it built for more strength. But it's your truck so you need to do what meets your needs and budget.

 

Just my opinion mind you, but I would keep what you have. Rebuild the AOD and add a shift kit. Later on rebuilding the 302 and adding lower gears will make a world of difference.