How do i go about bypassing the heater core in a '67?

a 4" piece of 1/2" pipe and 2 hose clamps should do the trick. Remove both hoses from the heater core at the firewall and splice them together with the piece of pipe. It isn't exactly rocket science.

You'll have to cut both hoses that come out of the engine. One hose is 5/8" and the other is 3/4" so 1/2" waterpipe might be too small and it might leak. I would use a 5/8" or 3/4" inch steel pipe nipple about 4-6 inches long from Home Depot or similar. and push the hose ends up over the ends of the pipe and clamp them, that way you'll know it's a tight seal and it won't leak when you build up pressure in the system once the engine reaches operating temperature. clamp both hoses to each other with the pipe as a union. Now, You should have an external vacuum or cable operated heater control valve located on the heater box on the firewall. This valve lets coolant into your heater core and keeps it out in the summer or whenever you don't have the heater on. One of your heater hoses goes into that valve first before it enters the core so remove the hose from where it goes into that valve and splice it into one end of the pipe or forget the pipe idea and you might be able to just splice the other hose that goes directly into the core right into the tubing on the other side of that valve using the valve as a splice, then you shouldn't even have to buy any pipe or extra clamps at all. The other heater hose can be cut almost anywhere as long as it will reach the other hose end. .The water/antifreeze will then circulate from the waterpump back into the engine until you get your new heater core.. good luck, Kenny/MrBlueOval