Nominal wheel width for 6.00-16 was 4 inches in that time frame - some earlier tractors used 3.5 inch width (measured wheel inside bead to inside bead). 7.50-16 nominal wheel was 5.5 inch - some at 6.0 inch.
If you have original wheels they probably would be 4.0x16 - possibly 4.5x16 and all is well. And yes the tri-rib will steer easier than truck tires.
If you have wider rims, the 6.00-16 will work up to 5.5 inch without a problem but they don"t look right on my MM RTI in my opinion.
You can probably find used but watch for bad rust and in any case make sure of bolt pattern and hub hole diameter. If you need new rims, I have had good luck with Hey Wheels http://www.heywheel.com/