The first OD for the 385 series was the E4OD which is an
electronic trans.  If you can find one for a 1989 - up which
I believe was the first year available, it will bolt in with
the correct parts and mounts.  You will need the electronic
controls and harness for it.  IIRC, the early controller was
a separate unit while the later models became integrated
into the ECM.  Bauman Brother or Level 10 or some others can
probably provide a controller and harnes - it won't be
cheap.

The alternative is to install an auxilliary gearbox from
someone like Gear Vendors.  This is not cheap either but, is
easier than a trans swap.

I suspect that you will meet or exceed the value of the
vehicle either way.  It may be smarter money to upgrade a
few years to a mor modern setup which will include the OD
feature alond with quite a few other efficiency
enhancements.

Lugnut

You might be able to gain a little of what you want without too big a
performance hit and for perhaps $500 by just changing the rear end
gears.