2' x 6' is well within the capability of 6x7. If you have the right lens for 4x5, that could be more than enough. If you have a 6x17 back for your 4x5, shoot roll film using that.
I tend to prefer negative film to transparency film unless I intend to project the results (or print Ilfochrome). A babbling brook may offer some extreme brightness ranges, so negative film might be best for that.
I don't have the format choices available to you. With my 6x7 equipment I would shoot two rolls - one Ektar 100, the other Portra 160 - and I would use the results that give me the colour saturation I liked best.