8.75 u-joints

how much power can the 8.75 small flang u-joint take with auto trans for bracket racing have 72 duster 340 4500 tci convertor motor 12.5to1 big solid cam 9.5 slicks last time i ran car was 19 years ago back then ran the factory 4speed with a little less motor and ran 12.40 all day long and never had any trouble with driveshaft at the time i did have the best u-joints installed that i could buy i think where spicer but just a little worried about braking something with this new motor thanks phil