I like the 5x7 ratio but have, at times, wanted to shoot a more pano ratio. So I finally bit the bullet and picked up a shen hao 4x10. It sports a 5x7 (yes 5x7!) back as well as the 4x10 back, and is quite light, really quite comparable to a 5x7. I recommend considering this option, since you say you are concerned about the extra bulk and it sounds like you might want ratios longer than 5x7. Consider that a 4x10 neg easily allows you to get the 5:8 ratio (and far beyond) via a rather modest increase in enlargement factor. Also I can report that 4x10 represents far less equipment weight than any 8x10 built to the same degree of stability. It's really quite surprising and I wish that I'd realized it sooner.
Another thing to consider is what lenses you have and what circles they cover, and what lenses you'd like to have as well. Covering 5x8 or similar with movements might turn out to be a bit more of a pill than you expect- just do your homework. The new view camera magazine has a nice list of image circles and such, I would recommend consulting that.
I think there might be a whole plate back for my rittreck 5x7; Joe Harrigan may have that back or some advice. Whole plate (6.5x8.5", right?) sounds quite similar to what you want, but what films would you use??? Anyway I considered getting longer ratios in an 8x10 and thought about making myself some inserts to go into 8x10 holders but after seeing how light the 4x10 is... no way. Would much rather enlarge a bit more and schlep less equipment. And I am avoiding exotic film cuts.