Why?

I had this same question when I first was planning the build on my car as well and found that the price to swap the 5.7 was actually pretty comparable to several other options I was looking at. A fairly vanilla 5.7 swap, a lightweight big block, a forced induction small block, and a small block stroker were all coming up around 5-7k for me (I put together a spreadsheet with all the different part prices). When I originally did the swap I went with a carb setup and had it in the car and running for probably 5-6k all together (I didn't get a computer or harness with my engine anyway). I later came across an awesome deal on a Megasquirt 3 and have since converted back over to fuel injection.

One of the big reasons I leaned toward the newer engine is because of the potential and general advancements in engine design in the past 40 years. As many have mentioned, some very light work easily puts you into 400hp+ territory. Doing that on the older engines requires a fair amount of high end parts.

I'll be the first to say my gas mileage hasn't been what I was hoping (currently around 16-17) with a 4 speed and a 3.55 rear gear instead of the 20 that several have seen. However I also realize I pretty much doubled the horsepower of the little 318 that was in the car when I bought it and kept the same gas mileage, so that's still a win in my book. I'm sure there's probably a little left in the tune to help bump that up, but it's been running well so I haven't messed with it. My next project is a T56 swap to get the overdrive that I really want once I find the right trans.

Long story short, the 3G swap is now pretty much entirely bolt in, which was one of the main reasons I attempted it. I don't have the fab skills or tools to make my own headers and do significant sheet metal work, or I would have probably gone with an easier swap. The parts are a little more expensive than other brands (like an LS swap into an old Chevy), but to swap a plain stock 3G into an old car really isn't that hard. The main thing that seems to hang most people up is the electronics. They don't really bother me, but they are a black box for a lot of people. Unfortunately they just don't seem to have a good solution for this yet. I originally looked into the FAST EZ-EFI system. They make a kit that's designed to work with stock fuel injection, so it's essentially just a computer and a wiring harness that self tunes, but I believe you have to have GM sensors, so you might have to adapt a couple of things.