We did it to our '86 sierra supreme, new stringers, floor, everything, because the old ones were rotted out. It took the majority of the summer, and a lot of fiberglassing work. There are other posts about it here with more info, try a search.
I am about to install a floor in my 83 Nautique. It takes a lot of time, but if you make all the proper preparations, it isn't that bad. I am using a have plywood, half fiberglass technique. Should be really strong.
I did one in my 88 Sanger. Pretty major deal. Check with the manufacturer cuz sometines they keep the blueprints and will sell floor kits in pieces... I know Sanger does for sure, don't know about others.
Floor kit sounds kind of expensive? If you are handy and know what types of materials to use, you can make a much stronger floor than what the boat manufacturers used back in the 80s.
Expensive? Not really. $300 for all the pieces in 3/4 marine ply encapsulated in resin including all the bulkheads as well, and all the fiberglass tape needed to laminate the floor pieces to the hull...and it all fit like a glove first time...