On the SWB series, you can un-rivit/spot weld the floor, and lift it out in one piece, which gives pretty good access to everything you should need -I would think it is the same on a 109, but someone will be along to confirm in a moment I did this to our lightweight, and it took about an hour to an hour and a half to get it all out -some rivits will need the end grinding off them unfortunately. Refitting was just pop riviting it back in place.
Looks like this with the floor out: *CLICKY*
And yes, a quarter chassis is probably the best way to go about it, assuming you can fine something to weld it to!