I think that 3.70 is OK. But, you will probably find that the 3.50 are more of a good for everything ratio. I run 3.50 in my 68 FB and they work good. I can still cruise on the highway. But, you'll be turning 3000 rpm at 65 mph. Up or down alittle depending on tire size.
Last year I removed the C-4 and installed an AOD transmission. What a difference on the cruise rpm now. I did change the convertor to an Art Carr convertor without lock-up. The O/D pulls the rpm down about 1000 rpm at cruise.
2005 F150 SuperCrew "King Ranch"
1986 LTD II
1964 Corvair Monza