You know the boys at N2RM have a lot of experience with them. Specifically you might contact N2NC (John Golomb). I'm not sure of his email address though.
Also, you might touch base with Ron Hooper, W4WA. Ron did this for 80 meters and I would think his experience might translate well to 40 meters.
My personal advice is to do it if you can get the elements horizontal. If you do inverted-V type, it will be more of a disappointment I believe. I think you would do well with 2 elements if they were both flat.
73 & GL
Bill