Help! Bumper Troubles.

[GreatOldOne]

Guys,

I'm having a real hard time fixing the rear bumpers to the chassis. I've just spent the afternoon drilling holes (which I assumed where straight) into the chassis and the bumper tube (which I tried to keep level by clamping it steady) - only to find that after tapping the bumper tube and bolting it up it all goes to cock, and out of true.

I've tried three times on both sides now, and I just can't get them to sit level. I've even tried using ali shims, and that didn't work either.

Any tips? If I drill any more holes the chassis will look like swiss cheese.