Finishing welding?

[Airhead]

Hey all

At what point in the chassis build is it potentially safe to finish off some of the welding?

The reason I ask is that as I am using an MX5 motor and box there is limited info on mounting and therefore tunnel dimensions etc. I will very soon need to move the chassis off the build table and over the engine to see where the clearance issues are but am reluctant to do so with it only tacked together... A logical point would be following the SB and D pieces are installed but before the rear suspension mounting tubes?

Matt