Re: engine size and fuel economy
Hi!
The "350 or so" cubic inch engines (~5.7 liter) aren't that bad for fuel economy, relatively speaking. You'd be deeper (and longer) into the gas pedal accelerating with a V6 versus a V8, so it's a wash either way as far as fuel economy goes.
Since you're looking specifically for a tow rig, you can find a clean, well cared for, fuel injected, 4 wheel drive late 80's/early 90's 1/2 ton pick-up with 100,000 miles or more for around $1200-1500. Even a mid/late 90's extended cab P/U should be found for less than $3000. Chances are you'd need to install a transmission fluid cooler right away, since most trucks didn't come with one from the factory when new (unless they were ordered with a "towing package").
I'm very new to towing/boating, so my advice should be taken with a grain of salt! But, I'm a "car" guy, and my experience is the V8 engines around 350cid are good. The "smaller" V8's, like the 302-305-318 are fair, but have to work harder than their bigger displacement brothers to move the same load; which in turn reduces their efficiency.
Like KETELS said, "economy", "trucks", and "boats" don't work in the same sentence. I'd find a nice, clean, '90 or so 4X4 P/U with a ~350 for a little over $1000. Good luck!
P.S. My post refers to OLDER trucks which is the subject of the question. The "newer" 2000 and up 4.3 - 5.4 liters are great, but would be out of the price range of the search.