The 64 Impala is arguably the most desirable vehicle to turn into a low rider.  This low-riding, all-foam, salmon fly is basically an extension of my El Camino golden stone, another fly designed to ride low in the surface film.  Bumping the size of the El Camino up to a #4 adds quite a bit of dubbing to the fly, which soaks up a little bit too much water, weighting the fly down too much.  Replacing the dubbed underbody of the El Camino with more foam on the 64 Impala easily negates any buoyancy issues related to  the increased fly size.  During the first few days of a salmon fly hatch the fish will pretty much eat any fly that's big and floating.  As the fish gorge on the huge insects they become more selective as to what they'll eat.  During my 6 years guiding the salmonfly hatch in the Gunnison Gorge I found that low-riding, less bushy patterns will continue to consistently pull fish, even though they've gorged on salmon flies for a few days and kind of lose that "reckless abandon" that they have during the first few days of the hatch.