Anything from a Schneider 75mm f8 SA upwards will cover 6x17 on a budget , I use a 75mm f8 SA as my standard lens on my 617 Gaoersi and it's perfect fot my uses.

I'm not sure a 6x17 back will work on a Speed Graphic, there's the frame part for the shutter curtain that's not there on a Crown Graphic and it'll crop the edges with wide angle lenses, not sure what FL that'll begin though.

You have given me some thoughts though, as I have a spare Super Graphic front standard - already modified to fit any Speed Graphic - and a load of Graflex parts coming, enough to make a great 6x17 field camera