I assume you're talking about the 351? The 350 is a GM engine. I suppose you can do it but you won't get the improvement you're hoping for. I've had trucks with both engines over the years and found they were quite close. The two trucks I owned with manual transmissions (1 with a 460 and 1 with a 351) were slightly better on fuel than the auto. When empty the 351 (may) have gotten 1 to 2 MPGs better than the 460 but I don't think it was ever better than that. When towing they were nearly identical because the 351 had to work so much harder.
What kind of fuel economy are you getting now? I think your money would be much better spent going through your engine and drivetrain to make sure everything is in excellent working order. Spend the rest on gas.
If i remember correctly the 4.9L I6 (300) the 5.0L (302), 5.8 (351W) and 7.5L (460) were all options in back in '96. So the short answer is yes its feasible. Now sourcing all the parts 22 years later may be more of an issue. By the time you spend all the time and effort to do the swap the gains may not offset what you put into the transition.
I had a '95 F150 regular cab short box with a 5 spd back in the late '90's early '00s and I seem to recall that fuel milage was around 12 in town and low teen's on the highway. I also thought it towed better then any of the other V8 options the F150 had at the time (the 460 was only available by that point in the F250 and larger trucks).
I'd much rather do a Diesel engine swap. Downsizing for the sake of it doesn't necessarily reverts into a mileage improvement, even though so many people simply look at displacement and amount of cylinders as the most determining aspects regarding fuel-efficiency while not considering power and torque RPM bands and differential ratio.
I owned a 1989 Ford f250 4x4 reg cab long box w/ 460 automatic average 9.5 -10 in town, in winter w/idling 7 -8. Bought a new 1992 same spec w/ 351, 5 speed same fuel mileage in town, it was hiway mileage that improved around 15 - 16. Had 2006 f250 4x4 w/ 6.0 diesel, auto,& same cab config mileage was no different in town, hiway was 16 -18. Had 2008 f250 ext cab long box w 6.4 diesel town mileage was little worse than the 06 because of regens hiway 15-17. Had 2011 f250 6.7 diesel ext long box city was 7 -10 winter, 10-11 summer, hiway 15-16. I now have 2018 f250 ext cab long box w/ 6.2 gas have been getting 10.2-11.3 this winter & gets 14- 15 hiway.
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