stringer swap come on anybody??

[full stringer]

hello gentlemen I have a 800 stringer full mechanical outdrive the gear ratio in the upper unit is 21;16 recently i encountered a problem with the lower unit vertical shaft, it broke. i found a good deal on a 800 stringer whole outdrive for cheap but it is a 21:20 upper ratio used commonly on v-6 and 5.0 litre v-8 i know the lower unit will be no problem but the upper is my concern and my engine is a 5.7 litre v-8 question is can the upper unit be used and if so would i need to increase the pitch in my propeller.