The cylinder heads were fly cut, so the original 1300cc-1600cc push rod tubes were too long to fit.
So I ordered a set of 1200cc tubes which should solve this issue. This time I ordered from Werk34, as I also found a nice oil pump cover as well, it is nickel plated on the outside and the surface towards the pump gears is almost honed-like. The shipment was fast and everything went flawless!
During assembly I discovered some clearance issues to install the spark plugs and the manifolds for the Weber carbs, so I cobbled something together which works exactly as it should: