In my experience, telephoto lenses have less coverage than non-telephoto lenses of the same focal length, so I'd prepare myself for blurry corners using that lens on medium format film. The smaller 2x3 Speed Graphic is the best (only) choice because of the need for a focal plane shutter and the need for a small flange-focal distance. Because of the telephoto design of your lens, you may have trouble focusing that lens at infinity on a full sized (4x5) Speed Graphic. I believe the flange-focal length for your lens is 44.7mm and the minimum flange-film distance on the small 2x3 Speed Graphic is around 30mm, so you should be OK. You may not be able to find a 6x4.5 back, but I'd just use a 6x6cm back and crop when printing (and you indeed may find yourself cropping to 28x28mm if you want the entire field sharp )