usually goes on top but no reason why you cant put it on the bottom
heres a little trick drill the hole where the pin will stick out on top then you can use the pin as the filler metal either by tig welding or oxy acetylene welding. fuse the pin to the brake and get off of it
make sure to use a heat sink on the inside of the barrel and use a wet towel to rest the brake on to use as a heat sink for the brake with the pin facing up so uou have a good position to fuse the join, a wet barrel mop will work.
file the weld flush after allowing to cool