Cause I've always been curious, are there big differences when it comes to economy of a 70s or 60s 302 vs. the 5.0 motors? Is one better than the other?
Probably not a vast difference, but the later roller motor will have less rotational friction and more efficient heads, so I'd expect marginally better fuel economy from a 5.0, all else being equal.
In stock trim definitely a huge difference. Once you put a crappy old carb. on a 5.0 it evens the playing field. EFI is more efficient hands down.
The 62 - 69 engines had higher compression, straight up cam timing and no emission controls to make mileage suffer. In 1970 things began to change for the worse. Compression dropped, cam timing was changed, carbs got leaner, timing was retarded, and the EGR systems were used to reduce emissions. Most of that stuff hurts mileage and power.
