If it's simply a head gasket, it can be replaced, along with other gaskets that will be replaced along with doing the job, for less than $65 if he does all the work. I might use a NAPA Cylinder Head Gasket Kit that includes all the gaskets you will need to remove and replace to replace the head gasket. Part number FPG HS7276B, $61.99 online price. I would feel pretty confident that if the truck has been cared for in the past, that it is just a matter of a head gasket that has deteriorated and is allowing coolant into the oil and also possibly into one or more of the combustion chambers. Pull the spark plugs and examine them. If one or more of the cylinder head combustion chambers is getting coolant into it, the spark plugs from those chambers will look like they have been steam cleaned and will have a very different look (they will look new) from the other spark plugs in chambers that have not had coolant leaking into them.
Anyway, that's what I would do if I were in your position. I'd replace the head gasket and hope for the best. If I were still getting coolant into the oil after the head gasket replacement, I might try using one of the chemical compounds to seal the crack/cracks in the head or block. Although I have never used it, I might try a product called Steel Seal. I have friends who swear it works. I personally wouldn't use it to try to repair a head gasket, I would first replace the head gasket. But if I got the new head gasket in place and still had a leak, I might try it before considering finding another motor. Here's a link....
I might take the head to an automotive machine shop and have it magnafluxed while it is off for the head gasket replacement. If the head isn't cracked, great, bolt it back on with the new head gasket in place and you're probably home-free.
If the head is cracked (must be cleaned up and magnafluxed at an automotive machine shop to determine if cracked or not), then the least costly way to fix it would be to find a head from a Chevy 216 in an automotive recycling yard (boneyard or junkyard in rodder parlance), have it magnafluxed for cracks and freshen up the valve job. The cost will be determined by the prevailing costs at the boneyards and machine shops in your area. This could be a 216 head from a car motor or a truck motor, either should work, same basic motor was used in cars and trucks from 1947 through 1953.
If the head isn't cracked and you have tried a new head gasket, then the culprit is the cylinder block and that means disconnecting and removing the entire motor. At that point, I would call around and find another whole motor, preferably one that could be started and run to verify its validity before I laid out the cash for it. I might expect to pay $300-$500 for a good running motor in this area. It would cost waaayyy more than that to totally rebuild one.
But my best experience tells me that the head gasket/Steel Seal will be the fix. Total cost, less than $150.
Others on this board may have alternate suggestions for you. I'm just telling you how I personally would proceed if I were on a tight budget. Head gasket first, then Steel Seal, then pull the motor and find another motor.