Updating 1950's hydronic system

DavidJ

My 1950's house had a single zone with each radiator fed off of 1.5" copper pipe loop. The loop has flow reducing T's that feed each radiator and return to the 1.5: line. I've had my boiler replaced with a Viesmann. As part of this upgrade I had distribution system set up to serve three zones. Each zone has its own pump and thermostat. After the pro's finished with the boiler and distribution install, I have been working on plumbing the zones. Two of the zones are in the basement and went in easy using pex pipe. I retained the cooper 1.5" pipe loop to serve the main floor.

Here is my question at last.

In order to simplify the distribution to three radiators on the main floor I am thinking about cutting the circulation loop and installing a manifold to feed three radiators with the return to the loop using an identical manifold onthe downstream side. The cut in the loop and manifolds would be just before the end run back to the boiler.

Other facts:
The manifold would feed three 1/2" lines.
There are 8 fin-tube type radiators on the main floor loop.
Each radiator is about 30" wide.

Is this a good idea or would I be throwing a constriction in the loop that may screw things up?

The pro that installed the boiler thought I should just cut out all of the copper and run 1/2" pex so all of the radiators are in line rather than being fed from a loop.

In retrospect this may have been much easier.

David J