3.55's will increase your RPM's throughout the entire driving range. The car will be quicker. You can do a little figuring on one of the rear axle/tire size/RPM/speed calculators. Here is one: Tire Size, RPM, Speed, and Differential Ratio Calculator
Typically, the factory 14" tire size was about 26" in height. Your present 2.56 gear in 3rd gear, 26" tall tire, 70 MPH, shows your engine to be running about 2300 RPM's +/- depending on slip from the torque converter. Going to a set of 3.55's will raise the RPM in 3rd gear to 3200 RPM +/-. About 900 more RPM's.
I would not want my engine spinning 3200 RPM's for long periods of time running 70 MPH - personal opinion. I feel this is not a comfortable RPM. I think steady running at 70 MPH should be near the 2500 RPM range - again, my opinion. Higher RPM's equal higher wear rates of parts and of course higher gas consumption.
One way to lower your RPM's with these ratios is to go with a taller tire. Lets plug in a 29" tall tire which is a good tire height. You now drop your RPM's down to 2880 +/-, so a reduction of 400 RPM's. Still a bit high in my book for long highway cruising. Going to a 30" or 31" tire is do-able, but probably not the look you might be after.
So, next gear size down is 3.36. Let's go with the 29" tire again. RPM's are now at 2725 +/-, still a bit high.
Next gear ratio down is the 3.23's which many Pontiac used. Let's go with a factory 26" tall tire AND taking into account the National Speed Limit used to be 55MPH! Your RPM's are now about 2300 +/- which is where the factory figured was the best RPM's for highway driving. Once we go to 70 MPH, your RPM's go up to just over 2900 RPM's - back into the uncomfortable steady cruising RPM's. Note that your present 2.56's at 70 MPH are the equivalent of the 3.23's at 55 MPH! So you are really in the right ballpark in comparison to yesteryear's speed limits and today's speed limits.
That all said, your optimum gear for 70 MPH cruising would be a set of 2.76's with a taller 29" tire to give you near 2250 RPM's +/- which in my opinion would be a waste of time & effort to change out only going up one range. 3.08/29" tire raises the RPM to right at 2500 RPM's which is acceptable, 3.08/26" tire is pushing it at almost 2800 RPM's.
Now you can see why the Overdrive transmissions are well worth the expense as you can have the best of both worlds, a nice peppy rear gear for 1-3 shifting and then drop down into OD for 70 MPH cruising comfort.
On my '68 Lemans, I was happy with the 3.23/29" tire and ran at 65-70MPH on the highway, 65MPH/2450 RPM - 70MPH/2625 RPM and I think this would be your best choice - my opinion.
If you change the gears out, it is not an easy removable center section type like the Ford. You will have to have a shop do this unless you have the know how on setting them up OR you can simply buy another rear end with the 3.23's in them and hope they are good after all these years. If I am not mistaken, you will also need the carrier center section to go with the "3" series gears and may not be able to use your "2" series carrier. But I found this on one of the blogs for Pontiacs, " I have a similar setup - 67 10 bolt rear, 2.56. Since it already had a posi, I figured it was at least worth a try with a 3.55 gear. I have not seen anything but 3.55, 3.90, and 4.11 for the BOP 8.2 from the gear suppliers. I got a Richmond gear and I had to use a Mr Gasket 10 bolt spacer and Mr Gasket 1" ring gear bolts for pontiac 10 bolt. The gear was still supported on the carrier shoulder w/the spacer so I figure it may hold up."
So you have options. Play around with the calculator and see what combo's you come up with. There are other calculators that can be worked for Overdrive RPM's as well should you opt for a late model OD trans upgrade.