Terry,
If you have truly & realistically repaired/replaced all damaged metal, used good welds, & have things properly aligned, there will be no sag when the other parts are installed. As Keoke said, the final panel fits were done with all of the heavy parts installed, but to my knowledge, the panel gaps don't change when the heavy parts are removed.
As John said, there should be no difference in sag, loaded or unloaded. What you are asking is to evaluate the quality & completeness of your repairs & make allowances for unknown defects, if there are any, ahead of time.
In my limited experience there will be no difference in panel fit with the drive train in or out on a solid car. A fair estimate would be to jack up the car one corner at a time & check for panel fit changes. Any change at all is a warning that underbody repairs are not adequate.
Some "pro" shops do a less than complete & adequate restoration & from experience, make some allowances for panel gap changes, they are sometimes able to estimate the degree of flex because they know what they left out in the under restoration. Other times they miscalculate. Nothing looks worse than a car which has had allowances for sag made by widening the top door gaps & then the sag does not happen. Door gaps can be equalized after the fact by trimming the door edges a bit if the gap is too tight & even adding metal to the door edges if the gaps are are too wide. A tremendous amount of extra work that most shops will not go through.
Your best bet is to build the rolling chassis & then final panel fit as Keoke said. Healeys are not one of the easy or cheap cars to properly restore.
D