OVERDRIVE TRANSMISSION

I just took my 68 Fury to its first out of town show and its very first trip on the freeway.
Now, I know I'm used to driving stuff with o/d trannies but it seems that the old Plymouth is haivng to rev a little too much to get to the 60 miles per hour mark.
Is there a MoPar 4spd o/d transmission that will swap into my Fury and replace the T/F behind the existing 318 ..............and what vehicle(s) should I be on the look out for as donors to get the easiest fit??

Cheers.
Terry.
Author: raindance654