Tunnel work process?

This may be a dumb question but...

Can a tunnel be fabbed and welded up with the motor/trans installed or do you take your best measurements after you've removed the old tunnel, yank the motor out and then install the new tunnel and hope you got it right? Would you tack weld your sheetmetal OVER the trans then remove the trans and finish welding with no obstructions? Do you need to weld from under the car or can it all be done from up top?

I'm thinking to build it w trans in place means no mistakes as far as final fit, but maybe harder to reach the joins?

Of course first I would trial fit first to see where there's interference.

Thanks