You must have one of those " chernobyl surplus models" damn that's rough dude
Cut The rot out is the only way ... But honesty if there is that much rot what does the rest of the car look like under all the bolts, screws, gaskets... Etc
2 years ago I did a 1988 suburban as a tow vehicle... It sat in the California mountains for 3 years and got dropped on in one place on the roof like that (barely sticking out of the carport) I had to cut out about another 1/4" of steel before I had viable plate to patch on to.