I played around with it a bit last night. I'm coming down with that flu-bug going around so I didn't get much done.
I looked and compared with the Toaster tank, and I think that teh reason it's not going in is that there is interference between the front top lip and the head bearing dust cover.. So I went back to Plan A and using a large socket as a dolly adjusted that front top lip to give a little more clearance. That worked-- it wants to drop in now, but there is some interference between the Hannigan fairing rear mounting bolts that needs to be addressed. I'll remove the fairing and continue tweaking the fit of this tank.
Such are the perils of progress... :)
'73 R60/5 Toaster