Not done a full chassis swap, but I have recently put a new rear tub on my '53 110............a lot of the nuts and bolts were already well rusted, so you need to budget for plenty of WD40, a nice 4" grinder, and lots of new nuts & bolts. I've also used stainless bolts for the doors and the rear tub to crossmember mounts.
Some of the rot will be hiding away, so you'll not see it until you get the 90 stripped down - don't forget to label everything as it comes off, even if you intend replacing it !
Best of luck.