Should be able to do it from all inside.Thats how I did mine.Cut old floor out with combination of angle grinder,drill bit,chisel.If you are carefull you can leave the fuel tank in.Rear seatbelt torx bolts will be a bugger to get out in one piece so get underneath and give the whole assembly a good dosing of penetrating oil(not WD40 cause its rubbish).Have a practice with the welder first on some scrap metal first and get the hang of it.Did mine with mixture of seam and plug welding.Boot floor crossmember supports may need replaced as well.Remember to remove or cover any interior bits you don't want burnt and cover the glass to stop grinding sparks from damaging it.Good luck