I do not think you should have trouble spanning 8′ with the 25′ cover. If needed you could attach the free hanging edge to a 2″x4″ . The domes and covers are freestanding, can be picked up and moved with a few people and have a stiffness and flexibility that would work in your favor. You would need to have a heater to melt snow off the dome during a storm but it sounds like you have thought of this. Yes, the 25′ is the top of the 30 and 31. Let me know if you have other questions. If you decide to use one please send a photo.