Looking through the Ez Chassis Swap info, their kits are for S10s. Those are technically rated for 6k lbs towing, but I sure would not try that.
Are you wanting to use it primarily for towing horses? Or occasionally?
If that is the primary use, you are probably better off with a newer diesel. That is the most efficient way to haul heavy loads.
If you are planning to drive it without the trailer most of the time, a 6.0/4L80e into the existing frame with a cam, a good tune, and good trailer brakes would be my recommendation. That would give you the power the pull it (with bad mileage) and the stopping power to stop it.
