A nice bolt in would be the A999 version of the 904. It's not an overdrive, but it does offer a lockup converter. As far as I know, there was no A999 for the slant 6 so you would still need an adapter.
and they (generally) have a lower first & second gear so you can run a more highway friendly rear gear without sacrificing too much giddy up on the street.
a reasonably half step to lowering the R's at cruise without cutting up a car and spending a grip of dough.