the 10 bolt will be fine unless youre gonna big block it an constantly hard race it with slicks on pavement or something else crazy, or if it already has a problem such as worn bearing or gears, loose pinion nut, etc.
there are lots of speedo gear ratio calculators on the internet and on billions of internet auto forums.,
do a google and you will find pages of them
or jut swap the speedo gears from the 3 speed onto your 400
to check for accuracy take it on the freeway and while driving 60mph exaclty, use a stop watch to time yourself between two concurrent mile posts
if you get to the next post before 60 seconds then youre going faster than 60mph and you need to slow the speedo down by putting in a larger driven gear, or a smaller drive gear
get to the post later than 60 seconds then you need the opposite thing.
hope i dont have that backwards