A "Brownie Box" can be added to most manual trannies that incorporates some degree of overdrive as well as resulting in much closer ratios throughout the range.
There's plenty of info out there on them. Just saw this the other day. It relates to a '78 GMC truck, but Browne-Lipe units are most common on diesel big rigs.
The only draw back I have heard from folks who have installed them is that they do require double-clutching, but that practice has been common on large, manual trans vehicles for at least a hundred years.