In classic bike racing 400cc is a nasty displacement: Too big for the 350cc class and just not beefy enough for the 500cc.
So what's the most logical thing? Go big. Unfortunately the XS400 series started out as an XS360 and was bored out to be more competitive. That said, a 15mm gudgeon pin is definitely lightweight, but not exactly durable and the conrod's small end is a bit flimsy to put it mildly and has a distinct tendency towards deformation.
Luckily a bit of research yielded some very, very interesting results. Now Suzuki's GS500 E is a pretty awesome parts donor, its 74mm diameter pistons being a nice upgrade over the stock 69mm items. With bore and stroke being rather close 52.4mm on the XS and 56.6mm on the GS500 E the pistons are actually pretty close in their overall dimensions.
Now a bit more internet research yielded an interesting result in the shape of FJ1100/1200 and XJR1200/1300 conrods to lower the pistons as the compression height of GS500 pistons is a tad higher than the stock XS400 items.
XS400 cylinders in the back, GS500 in the front.
An otherwise stock XS400 crank and two XJR1200 conrods in the front.
The money shot: The new piston is almost bigger than the bore in the cylinder, with the sleeve removed!
The XJR1200 conrod on the crank.
... and installed.