I got the 60 Ah pack in parallel down to about 2.97 volts and decided it was time to connect it to the parallel pack of the 40 Ah cells. It went fine, no sparks or nothing. Well, some sparks, when the connecting plus cable hit the negative, but that doesn't really count. Anyway, it is done now and the heater based bottom balancer is happily chipping away at the rest of the remaining charge.
I think I'll let it drain the cells for a day or two after it initially hits the target 2.7 V. They'll try to bounce back for some time. Perhaps not so much that there's so many of them, but there does seem to be a about 0.02 volts of difference between the cells at the center and very ends of the packs, which may also take some time to propagate.