Hope this helps. I put a Bushnell Sportsman 1' tube on my MKII, using a MGW mount from OntargetGuns. I think this mount will also work on your 678GC.The MGW mount will take any Weaver rings so you can use a 30MM Red-dot also Got my Red Dot from Natchez. Best advice is go online or make some phone calls to these and other suppliers and they will help you make the right choice.
With the one I'm using I've never been able to shoot better( got some vision problems). Ijust put one on My 10/22Magnum,30MM Bushnell Trophy.