HV Oil Pump/rear cap bolt clearance?

just been in the shed trial fitting the mains caps and arp studs. went to mount the Melling HV oil pump and it would not sit down properly. there was a big gap as the stud was too tall. even if I trimmed the stud the nut becomes the problem. even tried the old arp bolt and still no joy. took the washer from under the bolt and it was fine.
not to happy using one bolt. can the cap be milled and the bottom of the pump be ground down a tad to get around this.

cheers