It is useful if you can fit a GS tank since it is about 30cm shorter than the ST tank and allows a short rider to get closer to the bars and into the vee on the seat underframe. The tank goes straight on with only a water deflector needed between the tank channel and the top frame tube to avoid water getting onto the ignition coils (My bike is twin plugged)
I made my seat from an aluminium base trimmed to be a neat fit on the frame with a spike on the front to fit into the bike top frame tube (as the ST seat)and two lugs at the rear to pick up two threaded sockets I fitted to the new rear carrier front downtubes,. The seat base was then reinforced with fiberglass and trimmed to final size covered with a suitable foam and waterproof cover. Keep the seat as narrow as possible.
The next most important mod is to fit a surefoot type sidestand so the rider can park the bike on the sidestand without moving off the seat. Surefoot stands are no longer available in the UK due to European legislation ( a sidestand must now inhibit the engine running when down) I made my own stand but really should fit a microswitch to it that will cut the ignition circuit