Thermostat and housing...

[David Pollock]

If you want a stock appearance, use the original external bypass arrangement with the earlier water pump. Some replacement pumps will accept either as they have both the flat flange at the top as well as a 3/8" pipe thread. If you have the proper thermostat for this set up, it may be used without blocking the lower (in block) bypass circuit.  If you wish to use a modern style thermostat inside the older housing, leave the lower circuit clear but block the external bypass either inside the 1" hose or at the water pump flange.

My own 35 is like yours,  51 block, 53 head, but using the original 35 thermostat housing and early style (up to 50) stat.  The   Fenco rebuilders # on the pump is P47-3 and it can be used with either bypass setup.  In practical terms, the engine will run just as well without any bypass at all, but it is a good engineering feature allowing the water to circulate during warm up.

[JimCno]

I have a '35 Plymouth but under the hood it is a mish mash of components. The block is from a '55 and the head is from a '54. The block has an internal bypass for the water pump as does the water pump. The thermostat housing has the external bypass outlet. The water pump has the external bypass elbow. I know there are two designs of thermostats, one for the internal bypass and one for the external bypass.
Which  thermostat design should I use?
Is there a problem with having both and internal bypass and external bypass at the same time?