6.9 to 7.3 I.D.I swap
 

Im trying to decide whether I should buy an older pickup I ran across online yesterday afternoon. A guy has an 86 f250 he took the 6.9 out of and dropped a 7.3 idi in. He still has the stock 6.9 4 speed in the truck but says he needs a new belhousing to complete? Why would this be? Do the zf5 speeds have different bolt patterns??