If your goal is to bring it back to life with a decent freshly-scraped surface, you can ignore the rust, since it will be gone completely with the first few passes of scraping.
Even with de-rusting* you won't be sure about how flat it is and, likely, it will need some touch-up by scraping. Yes, scraping a 18x30" surface is not a quick job. However, to use it only as inspection plate, you don't need to fight till the last tenth f thousand and you don't need to get it to too many points per square inch, as long as the bearing is more or less uniform.
The added advantage of a scraped surface is that you can slide your gages without any significant stiction.
Paolo
*About rust removal: any acid is rather effective, but it has the disadvantage of keep digging into the metal: they don't stop at the interface between rust and metal. If you choose to use an acid nevertheless, I'd suggest you use vinegar, instead: a bit slower acting, much more controllable and not harmful at all. Use it outside, since the same vapor issues apply.
Molasses removes just the rust, stopping at the interface with raw metal. Similar, faster action is obtained using EvapoRust and similar chemicals.
Be aware that some rust converters based on phosphoric acid will leave a hard layer of iron phosphates, which is rather abrasive: a great idea if you want to paint over, not so great on a reference surface, where you'll slide some precision instruments.