I don't know what year/model you have but my 2005 E350's bolt sizes are:
Rear Seat Mounts: M12 x 1.75, I'm 100% sure about this measurement since I've ordered generic M12 x 1.75 bolts and they fit perfectly. (The lower seat belt-only mounts may be the same)
Top Shoulder Belt Mount: M11 x 1.25 (this is from measuring a stock bolt. The actual diameter measures about 10.85mm by my calipers and the 1.25 thread pitch matches that on a printable guide I downloaded) These bolts have an unthreaded shoulder of about 1.3cm long and 1.4cm in diameter to allow swiveling.
It probably doesn't matter for your needs but the stock bolts have mushroom shaped Torx 55 and Torx 50 heads, respectively.
Here's the printable metric thread guide I downloaded:
https://www.boltdepot.com/fastener-i...read-Sizes.pdf
Here's a link to metric to inch bolt sizes, but you're better off with the metric originals:
https://elginfasteners.com/resources...lt-conversion/
You'll probably find that you need to renew the threads in the floor with an appropriately sized tap due to rust, etc. Harbor Freight has some inexpensive tap and die kits.