3 speed would like lower rpm's hwy?

I would get some 15" rims to match whatever you have now and the biggest tire you could fit in the wheel well and put them on when you are cruisin' around. There are more choices in tire sizes in the 15" range than the 14's. I am thinking about doing the same thing because I can't stand the high rpm on the highway either. The tire that would fit with no problems on my 72 Swinger is 235/70/15. It is a little narrower but is taller than my 245/60/14. It depends on your rims backspacing for fitment. There are some good calculators on the web that you can play around with, just google tire size calculator and you will find something.