87 0r 88 GT 5.0 in a 66? possible?easy?

Fuel injection would be a lot of work. However, the 302 is basically a stroked 289, so for the most part, it will bolt in.

I assume you have the 2-barrel carb and intake on yours? Personally, I'd upgrade to a 4-barrel carb and intake. That will improve your cruising MPG, because the primaries are smaller than the barrels of your current carb. Grab the 5.0 for $300, but build it up before you put it in.

Grab it! Freshin the engine with new rings and bearings. Drop a Weiand Stealth and a Road Demon 575 carb on it and put a MSD Ready to run distributor in it. It'll make plenty of power and bolt in. You'll get decent mileage too. My modded 5.0 with 3.73 gears and a T5 gets 18-20 mpg with a strong highway/country road mix, and I'm not light on the exceleration.

I had several 3.0 and 3.8 V6 cars that got 25-26 mpg at best in the same conditions. For the trade off, the 5.0/302 V8 would have been well worth it and welcome.