ignition timing ?

A 13 degree stamped arm gives you 26 degrees of advance measured at the crank. You would only need another 5 degrees in the distributor to get to the 37 number you are seeing with the timing light.

I would try the protractor trick I mentioned earlier and measure the actual advance in the distributor. If you are mixing and matching parts from different distributors there could very well be some other changes that would make the "13" stamp on the arm invalid.

BTW, if I undeterstand you correctly you have a stock 318 and the only thing you have done is add a 4bbl. If that is the case why are you messing with the advance? There would be no reason to change the amount of advance in the distributor, just put in some lighter springs.