I think the actual image area of 8x10" film is 196x247mm, but I couldn't find the exact dimensions online, so I'm sure someone will correct me. You'll have to mask your 24x36mm image, film or digital, down to a 4:5 ratio, or 24x30mm. As mentioned above, if your camera can't crop for you, you could always put a mask on your LCD. Anyway, 196/24 = 8.167, so you're looking at a (16x25.4)/8.167 or 49.76mm. I imagine a 50mm lens will do just fine. Yes, this is considered "normal" for 24x36, while 16" is "long" for 8x10, but you're cropping the 24x36, and 50mm is a "long normal" when compared to the 43mm diagonal anyway.