You'll need a level and a plumb bob. Shim the bike at the center stand until the swing arm is level. Then remove the seat and run the same level along the frame under the seat. From there you can drop the plumb bob to see if there is misalignment from the top shock mounts.

I would assume the mufflers are gone. You'll only be able to check the muffler mounts with new mufflers. Parts like the muffler mounts are heated, straightened out, and re-painted.... you don't get a new sub-frame for that.

I may have a rear fender, tail lamp and turn signal bar, if needed.

Thank you, Richard!  Exactly the information I need.  Although I’m not sure what you mean by, “if there is misalignment from the top shock mounts.”  The string line will only show true vertical.  Are the shocks absolutely vertical when undamaged?  No front to rear or right to left angles?  I’m not sure what I’d be looking for, can you offer a bit more clarification?  I do have a /5 in my shop that I’m working on now, but no /6 available to use as a reference.  

If the sub-frame was bent from having the rear fender smashed, then it will typically no longer be horizontal. That will be very evident using the level along the top of the sub frame loop which passes under the seat.

But in rare cases, the sub-frame could also be pushed to one side... and still be "level" in the horizontal. This lateral movement will show up by dropping a plumb line (or a vertical level) from the top shock mounts to the lower shock mounts on the swing arm. The swing arm is VERY stiff. Of the 2, it is much more likely that the sub frame moved (got bent), rather than the swing arm. So the 2 mounts for the rear shocks (the upper and lower pair), as viewed from the rear, should be vertical.

The mounting distance between the pairs of shock mounts should be 340mm. And since the shocks themselves are comprised of a small, easily bent rod, you'll also want to check the eye-to-eye distance of the shocks themselves. Then if you can get the springs off, check the rear shocks for full stroke compression with no binding.

You're simply working from one known, unchanged area to establish if the next area is where it should be.

Hope this helps.

@wobbly Thank you Richard.  That helps a lot.  And I’ll keep you in mind if we need those parts.  Really appreciate your wisdom & help!

Come by in a truck. I have more body parts than I can possibly ever ship.

• @wobbly. I’d be happy to bring my truck, maybe even my trailer. But no idea where you live. I’m by Green Bay, Wisconsin.  You’re probably in Alaska or South America. But thanks!

Guys

After hearing nothing from the customer about his bike, I contacted him to see what was going on.  He told me his insurance company had totaled the bike, took possession of it and wrote him a check.  He went out and bought a Norton.  
Another airhead went to the scrap pile.  Sad

Posted by: @howard-stender

After hearing nothing from the customer about his bike, I contacted him to see what was going on.  He went out and bought a Norton.  

Norton's are a very strange combination of the new and the antique. The 'magic' is only skin deep. He'll be right back !!

I’m sure I wouldn’t have bought a Norton.  I would have looked for another Airhead.  I have a friend that has a 75/5 with a sidecar for sale.  He could have bought that one.  

I think that what happens when a vintage bike is totaled is that the insurance company sells it to a recycler if the owner does not want to buy it back from the insurer. That would keep the bike in the supply chain of spare parts. If anyone was interested, all you would have to do is ask the insurer where they sell these wrecked vehicles and try to track it down. I'm sure the insured could make an inquiry.  A friend who lost a bike to a deer strike saw his old bike for sale as a parts bike on ebay a few months later.