Okay, here's the fix for my La Pavoni Pre-Millenium Professional:
-Taking a Millenium version group 'plug' filed down to fit the back of the pre-millenium group, reaming out the thread area to 8mm for nearly all of the center hole, then filing that left area to 7.5mm I was able to tightly fit the dipper tube through that plug.
-Then filing a hole in the top of the dipper tube that extends inside the group cavity, cleaning all brass flakes off of the dipper tube and screwing this into the back of the group, as usual, you now have a group with hot water where steam used to be.
(This could be accomplished with a lab grade plastic cork too. That would be far easier than turning or filing down the millenium plug to size.)
Now when you raise the lever the piston rising will send water from the group back into the boiler, and as the piston lowers it will bring hot water from the boiler to the group. When pulling a shot starting with 92.0c I often see an overall rise of 5-6c. Dropping the PF into cool water, then locking into the group will typically drop the temp back to where I want it in the time it takes to grind and tamp the next basket. Mini pumps of the lever can be used to raise the group temp by 0.2c increments quite easily, and a full pumps will bring it up quicker.
Initial heating of the group does require several pumps of the lever with pausing in between, but that's the price I have to pay for an 8 shot La Pavoni system. Running the boiler at 1.1BAR is perfectly fine, as the group is temp stable, so now you can get better steaming for those cappuccinos.
I also modified the steam valve with a silicone hose, 3mm ID x 5mm OD, slipped over the end of the valve and cut flush. Now about 1/2 turn brings it to max, and the turn off is non-leaking. Got the idea here on HB with mention of power coating hole and bolt cover discussion, but didn't have those handy and the local hardware store had the small silicone hose in stock.
Thermo Probe Placement: Covered by velcro wrap for insulation from ambient. Naked k-probe.
Dipper tube cut show with files used to cut and cleanup the work.
Close up of dipper tube hole.
Steam valve with silicone tubing cover.