Hi Budgie,
Changed my rear crossmember and floor last month, once the floor is out, ( took mine out with a hammer and chistle ) all can be seen,
just grind off old mountings and weld new on, ( don't use a small mig welder, a 180 / 250 stick welder will do the job ok, use 2.5mm 3065 rods
on about 60/70 amps) the fuel tank should come off but if covered with a fire blanket it should be ok. but be carefull it's plastic. it took me a day to change mine, and i am a welder, so a garage may take 2/3/ days, it's not a bad job to do, if you need any advice pm me.
Cheers
The Welder.