2.76 to 3.91's SG

I have an A body 8 3/4 I'm saving for my Dart sitting in the corner of my garage. It currently has an open 2.76 gears with a 489 case. I got a lead on a set of 3.91's with "less than 500 miles on it." The guy says it has a sure grip and says it's "the big case (not 741)." How would I validate it to see if it's a 489 case? He has it listed for $500 shipped. I wouldn't buy it without seeing pictures of the gear teeth. Is this a good deal? Also, How bad would it be driving 3.91's on the highway at 60 mph with a 25-26" tire? My guesstimate says I'd be running around 3250-3500 rpm. The Dart has a 273 and a 904. Would like to go to a 360 and some 4 or 6 speed manual trans in the farther future. Is changing gears something that can be done in the garage? Thank you.