Cross member to Tub Bolts

Hi guys,
Have just replaced my '96 90's rear crossmember. Would have been a lot easier if LR hadn't routed the loom through one of the legs. Fortunately I didn't melt it when I welded the new chassis extensions in place!
Would like to replace the bolts that attach it to the rear body with stainless, but cannot work out what thread they are. I have metric and UNF gauges and they show that they are not one of these. Does anyone know for sure what thread they are? The size is 5/16.

just use whatever bolt will fit in the holes Think I used M8x20mm

I've just done mine. I used some M8x20mm. I got stainless ones from the local ironmongers, cost me about £4 including the nuts and washers.