I did 373 in my auto and it made a large difference in "fun driving" - quicker more nimble. Lost a little gas mileage - maybe 1 or 2 mpg. Then I did 410s and it was even better for driveablility. Again lost 1 or 2 mpg. Definitely makes the car more responsive and accelerate better, but will lose some mpg. With 410s and 127300 Comp Cams (springs), averaged 22 mpg at 75 mph.
I went from the stock 331's to 410's and it was a very noticeable difference, I'm not sure how big of a difference going to 373's would be, but I guarantee you will feel a difference, me personally, I would go 410's any day.
I went from roughly 1900 rpms with the 331's at 60mph to 2300 at 60mph with the 410's (5 speed auto here) and running at 80 mph I'm around 3000 in overdrive, my on board computer claimed I was getting 25mpg running (cruise control set) at 80mph at 3000rpms for about an hour straight.