Hi Gordy,
3.76 is better. That first gear in the 56 210 would have been 2.94. With a 3.70 rear first gear overall ratio would have been 10.9. That's super in a vehicle like the 210 where you won't be hauling weight. However your friend's 3.76 with a 3.90 rear is still going to be 14.66 first gear overall. All considered, I really like 3.38. 3.76 x 3.38 = 12.70 and that is almost ideal. I have 4.03 x 3.07 = 12.37 and you truly can not tell it from the stock 3 speed. When you get to fifth (with a 3.38 rear and .72 fifth) you'd have 2.43. If he is using 15 inch wheels and 205/75R tires, divide that by .97 to get 2.51. A stock 3 speed overdrive in a 1960 El Camino (came with a 3.70 rear end if you had the 235) will get you .70 (overdrive reduction) x 3.70 = 2.59 final ratio. 2.59 with the 235 was okay with the GM engineers in the El Camino (which actually weighed about 500 pounds more than the 1/2 ton pickup and also the station wagon.
With a 3.90 rear you'd have 3.90 x .72 or 2.80 and that will be good in fifth gear. In fifth I have .86 x 3.07 = 2.64 and it works fine. A tad over 2.59 is better in my experience. Your friend at 2.80 will have a bit of an advantage in more hilly areas. He can expect to be a shade over 2000 rpm at 60 mph with 28 inch diameter tires by my calculation. That's about where his maximum torque will be, also. Aside from first gear all should be fine.
Did the trans shop have any thoughts on the 3.90 rear gears?
