reducing hole size too big for helicoil

[wawoodman]

I have a 3/8 straight hole. How can I step it down to take a 5/16-18 shoulder bolt? It's already beyond the size of the helicoil drill, which is 21/64. If I use the helicoil tap in that size hole, will it hold?

I need a 3/8 shoulder bolt to fit through a bearing, but the securing hole needs to be reduced and threaded. I don't have a welder, or anything like one!