Rear gear advice

Looking for advice on rear gear selection. I have a line on an open 8 3/4 rear with 3.55 gears for sale, but I am unsure if 3.55 is the right choice for me. I have a mostly stock 340 and I am currently running 215/70/14's (would like to go wider in the rear). I only drive it on the street and do a fair amount of highway running at 65-70 mph. It wont see much abuse, but I want something stonger than the current 7 1/4, and I don't know what gear is in it now...

Advise on changing to a 3.55? I may be able to get a 3.23 from the same guy, but not sure yet...

Thanks,
Robb