Ok, that’s progress 😉
Matching fuel tank is very hard to find !
Best thing you can do is to catch a LTD or CSR fuel tank including top cover and (vacuum!) fuel petcock.
At least it has a PRI position 😉
Front fork legs may touch the front side of the LTD/CSR tank. Small correction of the max steering angle (Lenkanschlag) may be required.
Attached a picture of one of my 750Twins, currently under construction again. It has a B1 frame and the LTD fuel tank on it. Front parts like fork, triple trees/yokes, wheel, brakes, all from Z750E (4 cyl.)