Battery hook up

I have a 1974 m151 that is not running. I assume it is 24 volt neg. ground. If that is correct how do I connect the two 12v batteries to do this. And also if I have to buy new batteries what size would I need? 24,27 etc.

I use group 55. It is close to the original size. You will find two cables leading to the battery area. One is the chassis ground and the other leads to the starter switch. Hook the chassis ground cable to the negative post of one battery, then obtain a jumper to go from the positive of that battery to the negative of the second, and then hook the positive of the second battery to the cable leading to the starter switch. The two batteries hooked up this way, in "series", results in 24 volts being provided to the electrical system. Do not guess which cable is the negative chassis ground, because if you are wrong, you will instantly ruin a $250 alternator. Good luck and welcome to the site.