I have been scanning banquet camera prints from the 1940's (for website display) on my flat bed scanner with very good results. I scan them in 3 sections and then stitch them together.

For your 12x20's you might take a look at the Microtek 9800 series of scanners (B&H is a good place to look). With a Microtek 9800 (or equivalent) 2 scans should do it with plenty of overlap.