Marshall 8x10 would rock with that head, also.
Greenbacks are cool in fact they are my fav 12 inch speaker period but a Bassman will overpower 2 Greenbacks plus using a Bassman for a rock cover band I'm guessing he'll be using a lot of pedals and V30's will take to that better for most players than Greenbacks.
If you want a high powered "greenback", then Scumback is he only real choice.
The Greenbacks are rated at 25W. Two of them would handle 50W. The Bassman is rated at 40W. You should be OK, but you could go with a classic G12 which is rated at 60W. The G12 is made in a couple of configurations. 30W, 60W & 100W. Personally I would go with either the 25W Greenback(my favorite of all of them) or the G12H 30W. There are really similar in tone & response.
You really dig those Scumbacks huh?
Tried any of the new ones yet?
I have 6 on the way for a 4x12 and 2x12.
That's a lot of faith!
Not really, I know what parts Jim uses, and he has reconed enough old Celestions for me that I know his work.
You ever own any old Celestions yourself??