Norton, your going to confuse Dennish, all of my suggestions are Williams based options, and I have made power units out of dummy Williams for several years now, a few with the Williams upgrade kits, many more with components from Timko, and half a dozen with donor engines I gutted and placed into the unit I was upgrading.
ANY Williams 4 wheel truck with gears would work for the GP-38, I used ones from a genesis, as well as an F-7 to build 2 of mine, I had the frames and bodies from WBB and built a set for my overhead track. All you need is a set of side frames and pilots if donor not the correct ones. Williams is great for this kind of build, they seldom use specific trucks, instead sharing trucks across the prototypes.
Dennish, if you can find one, a Chuggington Brewster WBB works great, I built a GP-38-2 Santa Fe with one, I found the truck sides and pilots on ebay at a good price, and the truck/motors along with e unit swap easily. I put an ERR DC Commander in it for use as a command control engine for camera work on my layout. Watch the auction sites and be patient and for about $80 you can build it into a power unit.
I have also made several f-7 B units powered this way using donor f-7 engines that had shell damage, I just removed the pilot and they were exactly what needed. To make trailing engines rear facing, you just swap the engine plugs on e unit so it reverses the direction starts in.