The Pentax 645 frame is 56 x 41.5mm. So if you magnify the short axis by 4.896x to fill 8", the long dimension exceeds the paper by 0.79".

The Bronica ETR(S/Si) has better frame dimension at 55x42mm, and the Mamiya 645 even better frame dimension at 55x43mm, for filling 8x10 with minimal image loss