Overdrive trans or change rearend/gears

[fourspeedwagon]

I thought I read a thread about this before but.. I couldn't find it now.
My 68 has a three speed manual trans and I'm guessing 411 rear gear (maybe steeper?) I don't have a tach in it but it's really singing at 65-70 mph. I can't bring myself to cruise with it any faster than 65 for any distance. I'm not a great mechanic- as much as I love cars, trucks, bikes, tractors and own many, I admit, I'm just not great with a wrench. That said, would it be easiest to change out the trans for an overdrive unit or just find a rear diff with 355 (or so) gears?
My rig is a 68 2 wheel drive half ton suburban, mild 350. I think I'd rather have a different trans cause the 3 speed is also a pretty slow shifting unit but I can live with it.
What do you think? Thanks