The modded B17 is a standard one. A narrow would look better modded, since the Swallow is also a narrow. The only difference between a B17 narrow and a Swallow is the B17 has a ridge in the leather around the top surface, where the Swallow has none - see the pic of the black one.
As for guts, I agree it took a while before I took the plunge.
The B17 narrow will maintain its shape a lot longer since there is less leather to stretch in the sideways dimension. My black narrow is still very hard despite having been ridden a lot. Modding a narrow I think is fairly safe wrt to stiffness. My modded one is OK due to the flaps folded and tied underneath. But having support in the middle also helps - I at one stage had a piece of plastic between the flaps and the seat post clamp, supporting it. The side skirts are doing a very important job to maintain saddle shape and support; That is the reason why I cable tied my un-modded saddle skirts to the seat posts (see pic above) - these tended to flare a bit sitting on it, and the cable ties make a huge difference to keep the skirts away from the legs and keep the saddle stiff. The tension on the ties is so high that one snapped once.
The fork extension had a small amount of flex, but it was stiff enough for me to stand on the pedals and hammer up steep slopes while pulling hard on the bars. When I first built it it used to bother me a bit but I soon found that riding it I hardly notice anything at all.
Apologies for a major thread hijack, but t shows that you were spot on with starting it in the first place - don't we *love* these little bikes!!!!!!!!