Dustin - if I were you I'd hold off and just get a blade with the correct sub-frame. I don't disagree that it's possible to fabricate one but if you've never actually seen and had the correct one it's a real long shot that you'll make one that really works.
In the pictures of the one you just found the sub-frame has been reinforced for some reason and it looks terrible to me. If that sub-frame and blade had been mounted and used correctly it would not need that reinforcing.
Also, the blade in your pics doesn't show the lift rod. Narrow frame lift rods are approx. 2" longer than the wide frame lift rods. The narrow frame blade also has a slightly different lift rod bracket (it's bolted to that A-frame piece of the blade). It does NOT work on a wide frame tractor. It will hit the front of the tractor frame if you try to use it. Yes, you can fabricate a lift rod, and yes you can probably "bend" that lift rod bracket but I still recommend waiting and getting the correct blade with the correct lift bracket and lift rod.
If you attempt to fabricate the sub-frame, then you will also probably have to fabricate your own lift rod length, and modify the lift frame bracket. I just don't recommend attempting this. The snow/dozer blade for these tractors is a great tool. The correct blade for your 1250 is really quite easy to mount and attach (with a little experience). Fabricating something might get difficult to mount and then may not operate correctly with the tractor, and you could very easily end up bending your tractor frame, distorting your Quick Attach latch (making it difficult to attach your mowing deck, etc).
I strongly recommend getting the correct blade assembly.
Here's a diagram pic of the correct wide-frame sub-frame. You can see the 2 long bars are bent in and then curve up at the back. I have to say it's pretty hard to fabricate these with the correct curves, length and positioning.
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I wish Kraig, Oh Great One Keeper of the Photos, was around but he must be on vacation or something since he hasn't chimed in for quite some time. He's usually very good about posting photos to help out in these situation. I stole the one above from one of his posts from a few years ago.
I didn't find any other photos in the archives just doing a quick search so I copied these photos from eBay. This happens to be a 54" blade for a wide-frame like your 1250, which is really the same as a 42" just 6" longer at each end. You can't see the sub-frame details really well but it gives you a better idea of what the sub-frame looks like. It also has the correct lift rod and correct lift rod bracket for your 1250. If you look at this on eBay you'd see this same seller has a listing for a 42" blade which he says is in good shape but the sub-frame is bent. I would NOT agree. Yes, the sub-frame is bent to _ _ _ _ but the blade itself is also bent in the middle.
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I used to live in Montgomery Cty Maryland area outside DC (Bethesda, Rockville, Gaithersburg). I'd say you really don't get much snow MOST of the time but there are those occasions when you do get 3 feet in one day, usually Feb. or March. It would be nice to have the correct blade available so you can attach it quickly to your tractor, install some chains and go to town moving the snow. I do NOT recommend the 54" blade. It may be fine to move light snow, 2-3" deep but if you have 6-8" or more it gets harder for the tractor to handle that much. It's just grabbing to much at once. Yes, the 54" is somewhat hard to find and considered somewhat rare, and some collectors like to have them, myself included. It took me a long time to find one. I thought I'd like it better than the 42" but boy was I wrong. It was just to much blade for my 1450. The 42" really works ideal with this size tractor (although I will say 44 or 46 may be just a little better but IH didn't make those).
That's my story (dissertation) and I'm sticking to it. (And by the way, if you drove to CT you could get the correct 42" blade with the correct lift rod from my son for $150, but as PACub100 said there are places in PA probably closer to you).