A500/A518 Aftermarket Overdrive Ratios

The car is actually a 76 volare (I know not an a body). Building it with a 318 and would like to do the midwest drags with it next year where you drive to 4 different tracks and race on 4 different days. The car came with 4.56 gears built on a 742 case but even with the .69 4th gear online calculator has me at 2300 rpms at 60mph with my 28" mickey thompson radials.

Since there is not aftermarket ratios sounds like I would be better to swap rear end gears. Thanks everyone!!