I have an '87 D150 with a slant six that's totally stock, as far as I can tell. It has an automatic trans. I'd like to improve the reliability of the engine and have it make more torque for towing. What modifications/practices should I consider for towing a 4000 lb house trailer. I really don't mind if it's harder on fuel, as I would tow only once in a while, but I have one long towing trip coming up of about 1000 miles. Or would it be easier/cheaper to just remove the buzzin' half dozen and stuff a decent 318 down it's throat? Thanks for your input.
Steve

I think the /6 could pull that load all day, but I guess a v8 would be better for your use.
You have to get an other tranny too, if you swap.

Most of those trucks came with a 3.23 rear axle ratio. Your best performance improvement is going to come from changing the rear axle ratio to 3.55, 3.91, or 4.11. Then you'll want to change to a 2 barrel carburetor because the one barrel just doesn't flow enough air for the increased engine RPM.

Any engine "performance upgrades" you make will be killed by a 3.23 axle ratio.

Your 1987 truck has an ESA computer to control the ignition timing and the lock-up torque converter in the transmission, plus it controls the emissions stuff too. That's a whole 'nuther can of woms if you start changing carburetors and stuff on that truck.

But a 360 and a 727 into it and be done.

You might be time and money ahead by buying a truck already set up with a V8 and trailer towing package. Just a thought.

The v8 swap in a truck is mostly a simple bolt in...
You need that v8 tranny though.