I beleive that you are correct with the your statement that the WWII 6x6 "Duce and half" had a 270. The only 2.5 ton truck the US had was the CCKW, in a few different varients. I dont have all my info around me but that rings the bell. My collection and immdiate knoledge base is jeeps and Dodges, post WWII.
I am currently the VP of the So Cal MV group. After my own research, I beat the bushes on folks looking for a US vehicle that could have had the GMC 361 engine. I have not found anybody that knew or ever heard of a vehicle with a GMC 361. I am not saying that there never was one, but it was not a CCKW 2.5 ton truck and it was not a Half Track, that much is for certain.
I would love to hear from someone that knows, and I mean has documentation that the 361 was in such in such a vehicle, or a manuals etc. So far I have perplexed alot of people with this mystery.