Need help ring and pinion

[NicoSlam]

I read most threads discussing ring and pinion changes but I can't seem to get a good understanding of what is needed. I have a 69SWB with a 250 and muncie three speed on the column with 3.73 rear. I want to change R&P to a 3.08 for better highway use. Majority of my driving is cruising. I found a 3.08 RR in 1971 C10. Being that I want to keep the 6 lug and my axle is 1" shorter then a 1971, I can't swap rear ends. Can I do a swap from one to the other? Using the rpm calc, 70x3.73x1x336/31=2829rpm. If I go with a 3.08 R&: 70x3.08x1x336/31=2336rpm. Thats a 500rpm reduction at 70mph. Any opinions would be great.

thanks
louis