ih 756

There are several possible causes for water in the oil. Warped or cracked head, blown head gasket, cracked block or leaking sleeve o-rings or a cracked sleeve.
First of all, I am assuming that you removed the head and took it to a machine to be magnafluxed for cracks as well as the usual valvejob.
Second, did you remove the oil pan and pump up the radiator with a pressure tester to check the wet sleeve O-rings for leaks?
Third, You said the block isn't cracked. How do you know that? Did you remove the block and take in to a machine shop to be magnafluxed? The problem is often cracks are hairline that can't be seen when the block is cold. When you run the tractor and the block warms up, the crack expands and there goes the antifreeze into the crankcase.
Good luck. Hope this helps.

You have to buy or rent a radiator pressure testing kit. Inquire at your local auto parts store. Snap-On tools also carries the testing kit. I borrowed a kit from a friend when I pressure tested my IH 684 tractor. These kits are normally made for testing radiators for leaks, but they also work excellent for testing your sleeve O-rings. Usually your radiator cap will have a PSI number on it, if not just pump it up to about 14-17 PSI. With the oil pan off and the radiator pressured to approx. what it would be under normal operating conditions, antifreeze will be leaking out of at least one of the sleeves and dripping down onto the floor. If there are no leaks, then your O-rings are good. If your head gasket was not blown and your head was thoroughly checked by a machine shop and had no cracks, then it is most likely that your block is cracked. A sleeve could also be cracked, but it has been my experience that this usually results in compression being blown into the radiator (essentially blows all the antifreeze out of the engine). The block may or may not be repairable, depending on where the crack(s) are. Let me know how it turns out. I am curious to find out what the problem is.

Since half of your cylinders are leaking, the engine probably has quite a few hours on it. You can buy just the O-rings, but you have to do just as much work to replace them as to install new sleeves. If it were my tractor, I would put in a new kit in. Worthington tractor parts (Minnesota) has D-310 In frame kits (if the crank doesn't need to be ground) for $785.82. An out of frame kit is $1020.25. I think that's about as cheap as you'll find for an overhaul kit. Their phone number is 1-888-845-8456.
Good luck!