Steam boiler install

If there are no returns at the ends of the mains that means it is a counterflow system, the condensate returns to the boiler in the opposite direction as the steam. There need to be drips at the boiler to allow the condensate to return to the boiler. they need to connect separately to the return below the water line. the header should be piped as shown in the boiler installation manual and each main should connect separately to the header between the boiler connection and the equalizer.

That ~60 year old boiler is not piped correctly but it might have worked ok because older boilers had more space inside for the steam and water to separate.

You always size steam boilers based on the size of the radiation connected to it. You need to measure your radiators or other emitters and find the equivalent area from the type and size.

There will need to be some asbestos abatement to replace the boiler.

I agree it looks to be a counterflow system and already has the drips installed. The header is not piped quite right but it probably worked fine.

That should be fine. Pay close attention to pitch (twice normal). Vents are at the far end but drips / returns are at the source for counterflow. Look at EzzyT's recent installation.