8 3/4 geared to low. Whats compatable?

Before you go nutz buying stuff, why not jack up the rear end, and calculate the rear gear ratio. You will also determine if your 8 ¾ is a Suregrip or open differential.
Here is a simple method: http://www.slantsix.org/forum/viewtopic.php?t=36418

I have 3.55:1 gears in the Dart, and it turns 3000 rpm at 60 mph, this rather busy compared to today’s slow turning sub 2000 rpm overdrive equipped cars at the same speed. Back in the day this engine speed was considered normal, thrashing down the freeway.