This picture caught my eye, check the tubular cross member for the "A" frame.
The flat plates look quite corroded, best give the slap with a hammer test before you weld the floor in, you'll never do it from underneath once it is in!!!
Best of luck saving it matey, buy replacement panels for it and spend, spend, spend, it will be well worth it in the long run, time saved and less stress trying to blow holes, I mean weld in patches, Just cut the rot out, drill an spot it in place
PS.
If the cross member side plates have rotted through look
HERE to see y build thread where I tackled exactly the same.