There's not really a "correct" motor for the chassis, but it's true that the Pittman DC70 (RP77, DC77, etc.) is the typical motor used. The Pittman DC70 and 77 come in 6 volt versions, which are pretty good motors, and should have plenty of grunt for these chassis. Really, any inline open-frame motor will fit.
Martin, I like that modification of your chassis with the Tradeship motor; I have about 6 of these, with all kinds of different motors fitted: Kemtron, 36D, etc. And in fact, the one that I've run in competition is under an Allard body (Pactra) and has a K&B steering unit fitted to the front, because the Ulrich unit wouldn't fit very well! These chassis are fairly wide, especially if you use normal wider tires - they really came out at a time when skinnier tires were the norm. There's an old article in Car Model on fitting one of these chassis to a Ferrari 275 vac-form body, but it was a tight fit!
The other option is an open-wheel model, but that usually means carving out lots of axle space both front and rear. I wanted to use it on something like a 34 Ford, but same problem because of the molded in fenders.
Show photos when you get it done Isaac, curious to see what it looks like.
Don