It's been too long since I sold my M-135 in a fit of responsibility to remember accurately but I'm pretty much certain it had the tube still in place - so I can't tell you if it's important to keep it or not.
The 216 engines used in Canadian military trucks WW2 had a "steam relief tube" that ran from the rear of the head to the top tank of the radiator that was intended to ensure that steam pockets wouldn't develop at the rear of the head when operating on down slopes. Most of these steam relief tubes have been removed or blocked off post-war, either because they got in the mechanic's way or when the radiator was swapped for one that lacked the fitting for the tube to keep the truck running. Removal doesn't seem to have caused any problems that I've seen. (From my perspective today, that's a pain because they also often tossed the fitting that went into the head and also held the temerature sending bulb so it's now awkward to put the tube back in place for restoration.)
I suspect the tube on the 302 was intended to serve the same purpose as that on the 216.