The 300 I6 has the same flexplate as the 302(except the 300's has zero balance), same bellhousing pattern, too. The oil pan may be the biggest problem, in order to clear the x-member, but it's nothing some spare sheet metal and a mig can't fix. I'd most likely end up modifying the upper firewall so that the motor and trans could be moved back a few inches. That would help a lot with weight bias, anyway.
Think about it, though, a stock 300 makes its peak 270-290 ft/lbs of torque at 1600 rpm. With a 5-speed and 3.10s, you could cruise at about 65 mph at 1600 rpm, and since the motor makes so much torque, you could accelerate up steep hills, without coming out of 5th gear. With some head work, 4bbl intake, 390 holley carb, header, and higher ratio rockers on the 300, it could make an easy 240 hp, and 350 ft/lbs of torque. The truck guys on another website who specialize in the 300s actually get 3-4mpg more with all those mods done over the stock 300.
I seriously think 30 mpg highway would be possible in a light car, with substantially faster acceleration than a mild 302 would have. The 300 is no joke. But as I said, a II might actually end up requiring too much fitment to make it worthwhile, so a fox body coupe could end up with a 300 instead.
P.S. the 300 only weighs 560 pounds, ready to go, so weight isnt even an issue.