My recently renovated 1964 Spanish Gaggia, works good and brews a fairly good espresso, but it seems to take forever for the temperature to transfer from the boiler into the two groups.
After two hours at 115 deg.C boiler temperature, the two groups barely measure 70 deg.C.
It takes a lot of flushing to really heat up the two lever groups.
During heat-up the boiler is filled to 2/3 full on the level glass.

Between the boiler and each lever flange I have installed a 3mm = 1/8" thick gasket (klingerit)
Could it perhaps speed up things a bit if I changed the gasket to 1.5mm thickness = 1/16"

You could split one gasket in half with a sharp knife, and compare the two groups. See which gasket thickness you like best.
Maybe you can fiddle with the heat flow adjustments (they are heat flow adjusters right?) on top of the group-boiler connection.
Also, keep in mind this machine was designed for back-to-back espresso pulling.
Your project turned out really nice!

Thanks Bluecold;

I like your idea to modify one gasket first and see how things go, if all things fail, I still have some very thin (soft) copper sheetmetal laying about, may even try that.

And yes, this type of leverhead has the needle valve, ideal for fine tuning the water flow to the brewhead , I did also use the valves during the final boiler pressure test, with both headers in place.

True it was a nice project, thank you for your support.