Be careful - I've heard plenty of Blackface LA-4's sound like crap. You can always have them re-chipped, re-capped, and overhauled, but just know that if you see one for sale, it's very likely it won't be ready for your rack. They're awesome boxes, but if it's been in continuous use since the 70's, it may need a little love.
And yes, you want Blackface, not Silverface, 'though Silverface is still damn good.
jb