Yeah, you'll have no trouble making a couple of great 312-type preamps with those cards. Be aware, however, that those cards have the 2623 output transformer instead of the 2503 used on the 312. According to api transformer info the 2623 can do 1:1 or 1:2 outputs, which makes me think they're trifilar wound, instead of the quadfilar winding of the 2503. So you will not be able to provide simultaneous 1:2 AND 1:1 outputs with those.
Practically this is a non-issue, however, as most people only use a single 1:2 output, which these will easily supply. I'd be shocked if there was much noticable difference in the tone.
Regarding the desoldering of the opamps, DO be careful not to keep the iron on the leads more than 5 seconds or so. If that's not enough to desolder (using a good quality desoldering braid), then let it completely cool before putting the iron on them again. Better to be very safe than sorry, as those opamps are basically like gold. API will sell you replacements if you send them a bad one, but they're like $75 each still, so better to be very careful with any that you have rather than incur more expense.
Even better, see if you can buy or borrow a desoldering gun (with a built in vacuum pump) with a large barrel tip (those 2520 pins are much bigger than typical DIP-style opamp pins). That would be the best for this type of job. I finally broke down and bought one at Fry's for around $120-ish. I've never regretted that purchase.
Also, regarding input iron, I suggest Cinemag 75101 inputs on those. Compared to the 115k-types often used on those the sound is much richer, silkier, and fuller, without being mushy or dull at all. Just my 2-cents.
That's a good find. Good luck with your bidding!
JC