Hey, this place supposedly has an R151...

http://www.likekindqualityautoparts.com/

Medford, LI, NY

if you can get it for under $450 alone, or under $600 w/tcase, and they guarantee it, GRAB IT!...(Make sure you get the bellhousing too, though!)

IIRC, the G, W, & R series are all basically bolt-ins for each other...

I would at least hold out for a W56...they are probably almost as common as the G series, if not more so, since they were a replacememnt for the G series in 88...Shoudl be just about the same price also...
& it has slightly lower 1st gear, as well as being a bit beefier...