I welded half a 91 florpan to a 96 rusted out and it took a weekend to do it all, from trunk to front firewall, all the holes welded up with donor caprice roof and quarter panels metal
Its doable but there another thing you will have to consider.
If the floor is that gone, it means ALL the bodymount support plates on the body are rusted out and failed. thoses are not easy to fix and even most body mount bolts will brake out.
3 years later when I got in a fender bender and scraped the car that got floor welded, when cutting the car in half i was shocked to see that the whole car was holding barely to only the middle body mount on each side.
Most of the body frame rails will be far gone in rust too.
Welding it will be band-aid it for a while but its too far gone to my opinion.
The car I'm driving now got off the road in 2001, and i fixed all the rust floor issue with stainless in autobody school, it was quite involving.