When I purchased my Linhof Technorama, I received a full set of Cokin X-Pro square filters, plus the filter holder that screws to the lens, and I have been very happy with the results. The Schneider Super Angulon XL 90mm f/5.6 is pretty huge as LF lenses go (105mm diameter front element once the fall-off correction lens is screwed onto the front) and I have been very happy with the results.
(as you know, the 90mm lens is extremely wide on the 6x17cm format, comparable to your ~60mm on 4x5in)
Here is an example of the Cokin X-Pro 2-stop Grad ND, in the worst possible lighting (pointed right into the sun) and there is only a tiny bit of flare, and the colours stay quite neutral:
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djkloss, to answer your question, yes you can either use a "tobacco grad" filter which wil give a yellow cast as well as decrease exposure, or you can combine a warm-up filter with a Grad ND. With the Cokin X-Pro set, you can slide two filters plus a polariser into the holder, and I've never seen vignetting.