Hi, wanted to let you know what we found when we got the floor up. The floor joists are mostly okay and spaced correctly, but the original owner who built it cut into the header that runs along what used to be the east outer wall of the house and undermined the floor's support. He did it to run venting into the addition. It looks like I'll be able to return some of the 2 x 10s I bought to replace all the floor joists. After we get the header repaired and check all the joists (we've only got half the floor up; we spent yesterday taking down the drywall and pulling up the old wood floor plank by plank), we're putting down a vapor barrier over the concrete (it was built over a carport), then insulating and putting down new plywood. I'll send pictures when done.
Thanks, Bruno! I really appreciate your advice and support.