The 1000.4 is only acceptable for mpeg-4 receivers. The 311 is a standard definition mpeg-2 receiver. So if you wish to keep the 311, or any other Dish receiver incapable of mpeg-4 decoding, then forget about the 1000.4.
The DishPro Plus Twin LNB has two outputs and one input. So you cable up your wing dish LNB to the twin's input. It has a built-in DishPro Plus switch capable of feeding two dual-tuner receivers such as the 612, or a 612 and a 311.
But if you're willing to do anything regardless of cost, just call up an installer and have him do whatever it takes. He can't give you a 1000.4, though, unless you never want to plug in that 311. He can easily swap the legacy twin for a DishPro Plus twin, install and align a wing dish for 61.5, remove that old SW21, and get your 612 going.
The 612 also has an ATSC (digital) tuner capable of receiving your local stations for free. You might have your installer also add a rooftop broadcast TV antenna as well. That too can be combined into a single cable to the 612, but that's a little bit confusing so an installer might be best.