I use S2 base sections upside down and I use 38mm hose. I find the hose cuff is perfect to push into the base section and the 38mm hose pushes just right into the top tube whilst fitting over a 41mm nozzle. You may have a slight problem getting the hose to fit the vac itself. I pulled the inner out of the Nilfisk connector ('cos it's rather small) and found a cuff knocking about that fits the tapered Nilfisk one.
There are cuff adapters about that can go from 38mm to 51mm.
Most guttervc manufacturers are using 51mm. Personally I can't see the point if the nozzle is far smaller and the tubes too. also the Nilfisk inlet is smaller than ideal but it works 90% of the time. Polarvac do an aftermarket cyclonic inlet that you may be able to fit but it's too expensive in my opinion.
I've had my set up for years now without major problems up to 50' with my bigger vac.
I bought my tubes when they were just over £50 each - I think they are three times that now for carbon