I own a 1999 Ford F-150 4x4 extended cab with a 4.6L V-8 and an automatic transmission. This is my daily driver and has 245,000 miles on it. I run 305/70R16 tires. I pull a trailer from time to time but enjoy mild off-road adventures as well. I have a 19,000-mile engine waiting to drop in and want to rebuild the transmission when I install the engine. I am debating on installing 4.10 gears or a turbo system. I want to try and get as many miles per gallon as possible but gain performance as well. I haven't found many reasonably priced turbo systems for my truck, but Mustang systems are cheap and easy to find. Is it possible to make a Mustang turbo system work without major changes?
Another option is a straight axle swap. I have a Dana 70 rearend but no front axle. Is there a certain year that works best and is easy to find, or can I use any Ford front axle? Any advice would be helpful.
If mileage and power are the primary goals, I'd lean toward a gear swap. Your truck likely has somewhere around 3.50 gears, and with your truck's current tires being about 3 inches taller than factory, the engine is being forced to operate below its optimum powerband, which hurts performance and fuel economy. Swapping in lower gears such as 4.10s will help restore much of that lost power. And even though gearing axles is expensive, it's still going to be cheaper than a turbo kit. There are a couple of turbo kits on the market for your truck, along with a few supercharger kits, but these can run upwards of $4,000 not including installation. A gear swap will likely run half that amount. It's probable that you could make a Mustang turbo kit work, but there will be some significant differences in underhood packaging, meaning that you are looking at a lot of plumbing modifications at minimum. Also keep in mind that turbos and super-chargers aren't going to do a thing for mileage, and most of them require running premium fuel, upgraded injectors, custom or piggyback programming, and so forth.
A solid axle swap wouldn't be easy, but it is possible and several people have done it with an F-150 like yours. There are two basic schools of thought. One is to find a suitable donor front axle with a driver-side output to match your truck's transfer case and close to the same width as your truck's rear axle. If you can find one, a Dana 44 from a pre-1980 Super Cab F-150 with leaf springs would be just about the perfect candidate, as it would be very close to the right width and would be mostly free of brackets and castings that you would otherwise have to cut off the axle when fabricating new suspension. However, these axles can be very hard to find and the wheel bolt pattern will not match your truck's rear axle. The other school of thought is swapping in a set of axles from a similar year Super Duty. The Super Duty axles are a little wider than your F-150 rear axle, hence the need to swap both of them, but the Super Duty rear axle should have the tone ring and sensor that is necessary to keep your speedometer functioning correctly. A third option would be to simply have an axle built to your specifications, which really won't be that much more expensive when you consider the expense of modifying and regearing a donor axle. For suspension, you can duplicate pre-1980 F-150 coil spring suspension, fabricate your own four-link suspension, or run leaf springs. Each of these options has its own compromises in terms of ride quality and ease of fabrication. Regardless, a solid axle swap is a huge undertaking with a heavy amount of fabrication and custom parts involved, so you want to be sure you have the necessary tools and knowhow before diving into the project.