Found that the front of the base-frame fitted perfectly where just the day before a bit of metal had been welded across the two front suspension arm supports. Whoops. Then spent the morning setting up with bits of stick and tape (again). This allowed me to appreciate that a couple of diagonal braces leading underneath towards the nose would be needed, in order to support the front suspension in tripod fashion. Interestingly my intuition warned me not to weld the diagonal supports on (shown on the ground underneath the base-frame) where first chosen: putting the gearbox between them at the front showed that there was a problem: one side of the gearbox would hit the frame, so they've been moved over a bit. whew.