Canuto, welcome to the site. You'll find your way around and start to explore by doing SEARCHES for subjects and problems you are having. You will also find FORUMS to be helpful in addressing specific problems like ENGINE and DRIVE TRAIN.
Having put over 110k miles on a '49, in addressing your , "...which hose should I install the shutoff valve so no water will go into the heater." I have found that I need to shut off BOTH in a non-stock after market heater. The reason is that even though the flow cannot occur with a shut-off on either hose, migration of heat via the hose that is not shut off will occur.
You have a temperature control valve which should if operating correctly open only when you want it to. I would think a single shut-off on the other hose coming off the water pump would keep water out of your heater radiator, thereby keeping heat out of the cab..
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