The Marlin 336 used was a .35 Remington. It has the same bolt face as a .250 (within .006"). The 336 actions are all the same, from .30-30 up to .450 Marlin, (it was the Winchester 94 that required beefing up for the .375 big bore and .307/356). The only differences are the ejector port size and mag. tube hole. The 6.5 Jap would actually be a real natural for this conversion, and is one that I have given some serious consideration. Breach pressure of the .250 is 47,000 while the .307, .356, and .375 are over 50,000. I was also considering stamping the barrel .250 Marlin so as to give anyone pause that wanted to load it up with factory .250 Savage spitzer rounds. Anothe option might be fitting a Remington spiral magazine tube ala model 141.
In conclusion, The .250 Savage is short enough, low pressure enough, The cartridge stop will work, there are plenty of .25 cal. flat nosed bullets if I don't want to use it as a "2 shooter". The conversion has already been done at least once, and a rebarrel of a 336 is still cheaper than a Savage 99 in .250 . I want this rifle for varmints out to 200-250 yards, using 60 and 87 gr. bullets. I could load it to 25-36 A.I. velocities and have plenty of speed with lower pressure. Deer loads with the 117 gr. round nose should be effective out to 250 yards without straining the action. This cartridge should be nearly as effective as a .307 for deer and better for varmints.
I welcome any other thoughts about this conversion, as I am trying to find any issue that might be a show stopper.