I am running a Muncie M21 with a 3:31 rear end gear. I thought this combination would give me better gas mileage and lower RPM at Highway cruising speeds, but my 350 is still turning 3000 RPM's at 65 MPH. I'm not drag racing the car, so getting off the line quickly really didn't matter to me. It just feels and sounds like I need a 5th gear. Is this normal?
Any advice would be appreciated.

Thanks,
Woody
69 RS

Very normal. I'm old enough to have driven cars before over-drive transmissions took hold. I guess we just got used to it. Also, sound insulation today is 10x better than in the late 60's so you hear the drivetrain (which I love).

But 3.31's are pretty mild, and usually not coupled with a M-21 (close ratio).

Your best option is to go with a 5-speed with overdrive.

FWIW

True. Even with 3:08 gears the rpms will be up to much for decent
mileage. 5 or 6 speed

A friend of mine is running an M-21 and 3.31 gears in a '69 Chevelle with the same results. A little sluggish off the line and a little high on rpm's at cruising speed. I tried talking him into a TKO 600 RR. With the 2.87 first gear and the 3.31's it's like having the M-21 and a 4.30 rear in first gear. The .82 fifth gear turns the 3.31's into 2.71's. Best of both worlds in my opinion. I have a TKO 500 and 4.11's in my Camaro and would never go back to a four speed.

Has anyone had any experience with the riverside gear M21 overdrive kit? Ratios are as follows, 1st gear 2.20:1, 2nd gear 1.50:1, 3rd is 1 to 1, 4th position becomes .87 for a final of 13% overdrive out the tailshaft.
http://www.riversidegear.com/categories/muncie-4-speed/products/product-166.html

Borrow some taller rear tires and take it for a spin. Maybe you will like the difference.