351 rebuild mix and match heads?

Alright, I just had my 351 from my '91 SN rebuilt over the winter. One of the heads was cracked and the guy at the engine shop had one "laying around" that he did a valve job on. He told me that the casting numbers were not the same. But, it was a head for a 351 so it should work fine. From what I can gather, it looks like the head he sold me does not have hardened valve seats. I was reading some other posts on here that said if you don't have any ribs or bars on the end of the head, it does not have the hardened seats and i'll need to run a lead additive.
1. is this correct?
2. will that cause problems with my other original head that does have hardened seats?
3. is there some way the engine guy might have installed hardened seats when he did the valve job? (i'm kind of a rookie and not an engine builder. i didn't want to ask him that and look stupid)

Sounds a bit sketchy to me, its possible they are extremely similar heads, however it is also possible there are differences in combustion chamber, runner size, valve size, etc that to me would be completely unacceptable. Give us the casting numbers and we can do some look up.

As for bars on the end equaling hardened valve seats, thats not exactly right. The GT40s have 3 bars, and the gt40ps have 4 and both have hardened seats but my heads from 78 (which have very low hours and you can have for the cost of shipping btw) have hardened seats and no bars. However if I matched one of them up with a Gt40p or a GT40 I would expect bad things to happen, I mean it might work but its nothing I would pay to have put into my 91 SN.

On the third question, yes it is possible and with better machine shops standard proceedure to install hardened valve seats when a valve job is done on older heads.

Thanks guys. I'll have to track down those casting #'s. The guy did tell me they were different. But, assured me they were the same size and all that jazz. So, I hope he knows what he's doing.
On a side note. I also just ordered a new damper plate from skidim. It is the wrong size. I called Vince and he said they switched to a different style. It's totally round now instead of triangular, and the diameter is almost bigger than the flywheel itself. I was able to get it bolted on. But, it rubs on my bell housing and the edge of the starter. So, what the heck PCM.

So, I talked to Vince from SkiDIM on the phone for about an hour yesterday. Yes, it is directional. They make it pretty idiot proof by stamping one side with "towards flywheel". Moral of the story. Pray you don't break your flywheel damper on a '91 SN with a 351. The part I got is the "new" style, one-size-fits-all damper that PCM is now selling for all motors. This new damper ends up being the same diameter as the flywheel. With the extra width at the edges of the flywheel, my starter won't fit and my bellhousing rubs up against the damper too.
I was going to install this new one as preventative maint. to avoid the hassle later. Now I'm just hoping my old one hangs in there until end of life for the boat.