Cab & Bed Alignment

[HuntinNut]

Ok so I've spent years getting collecting and getting to the point where I could set the cab and bed on my frame and start putting my 66 back togather. I set the cab on and bolted it down and spent Sun cutting new wood and setting my swb on the frame.
I thought the cab was square or aligned on the frame, but I cannot get the bed aligned with the cab so I be leave the cab isn't right so I have a couple questions.

What's the best way to get the cab in installed correctly or where should i take my measurements ? From the few measurements I did tonight I believe it's off.

What should my gap be between the bed and frame ?