Scott C. -
The H42 was made in 1980 and beyond and I don't believe it was intended for use with the Quiet Line tractors (which your 1250 is one of a series - 800, 1000, 1200, 1250, 1450 and 1650) - Quiet Line production ended in mid-1980. The "proper" snowthrower for said tractors is a QA-42A or QA-36A.
That said, I believe it's possible to cobble it together to work. First of all, the pulleys on the QA snowthrowers also hit the latch bar - you will have to adjust the pulley into it's lowest position so it can clear the latch, then adjust it all the way up so you can put on the belt.
Secondly, I believe the pulley is meant for the 5/8" wide belts found on the red "x82" tractors, whereas the PTO on your 1250 is designed for 3/8" belts. You'll have to find a narrower pulley at a farm store or industrial supply house.
Finally, you will likely need a spring assist for your manual lift. They can be obtained from just about all our sponsors above, and are sometimes offered for sale in our classifieds.
If you REALLY want to have some fun, you can convert your 1250 to hydraulic lift. Perhaps not for the timid, but definitely doable with the right parts and a thick enough wallet
Also, as I say to all newbies, buy manuals! It makes life so much easier. Read the FAQ at the link above, too.