Re: 12" Greenback crammed into a Marshall Lead 12 (5005)
I own a 1983 Lead 12, GREAT practice amp and it takes pedals really well. The "clean" sound is pretty decent too.
As for the speaker, it has a 10" so trying to get a 12" speaker in there would be a tight fit. The stock speaker is actually pretty good anyway. FWIW, using an MI Audio Crunch Box into the clean input is total 80's Metal, and boosting the dirty input with a Tubescreamer is about as close as you can get to a cranked Marshall stack at bedroom levels, IMO. The only issue with the amp is the input jacks, they go bad every couple of years.
Cheers,
CJ