Roeland
The fact that your brake currently works with the bolts slightly loose says that it is possible for the shoes and cam to find a location that is correct. The trick is to find that position and then to tighten the retaining bolts down so that it remains correct.
About half way down page two of this link is a copy of the Royal Enfield instructions on how to do this. It is highlighted with a red bracket.
http://www.enfieldmotorcycles.com/forum/index.php/topic,12728.15.htmlTo paraphrase what it says, with the bolts
slightly loose so that the cam and its guide can move if forced to, assemble the backing plate & shoes with the drum installed on the bike.
Tighten the brake actuation lever as hard as possible to force the shoes and cam to a centered position. Then, remove the force and carefully remove the backing plate & shoes from the drum.
Without moving anything, carefully tighten the two bolts. The cam and shoes are now properly aligned.
When reassembling everything it is necessary to once again tighten the brake shoe actuator rod to centralize the assembly with the drum and wheel. Tighten the large nut on the backing plate stud which protrudes thru the swing arm flange and the large nut that is in line with the axle. (The chain adjustment requires checking before tightening these two fasteners).
Once both large nuts are tightened the brake actuator lever rod that connects to the foot pedal needs to be adjusted so the brakes do not drag when the pedal is not depressed.