fixing stripped carb bolt hole on intake

hey guys,

i checked out my carb base plate and if will fit a 5/16 bolt with no drilling...so keeping that in mind, id like to use a 5/16 bolt

now the threads that are screwed are 1/4"...what drill bit size would i have to use before i can use my 5/16" tap to thread it? its something id like to fix right now instead of having to order a part for it

thanks!
Matthew