Hey Scott, the first guy that installed hydronic heat in this house boxed in a few pipe chases along the inside walls. The original 1/2" black iron supply/return pipes were still there, so I just used the proper brass fittings to adapt them to pex in the basement. I don't know what they used for pipe dope in the 50's, but I was able to take 50+ year old fittings apart with 18" pipe wrenches.
On a side note, the guy that installed this stuff the first time probably knew what he was doing. I think the trouble started when the boiler was swapped in the late 70's, and the original convectors were replaced with fin-tube radiation...I'm sure thats when all balance problems started.