Dave,
Your thinking is very correct. Your best results are to replace a factory panel with a factory panel. Yes, it's a bit more work, but your results will be excellent.
When I replace a floor pan, trunk pan or any other panel, the first thing I do is blow out the spot welds with my plasma cutter and take the old piece out as it was put in by the factory. With a spot weld drill bit, you'll have to grind the extra layer off to get it flush and then plug weld the the new piece back in. Works fine - last a long time!!
Be sure to cut out more than you need from the donar car and take your time back in your shop and remove the excess by drilling out the spot welds.
You should be able to purchase a spot weld drill bit (looks like a very small hole saw) at any automotive paint supply store.
Once you've replace the dash, you won't be able to tell it's been replaced. Take your time and PM me or email if you have questions. I am the POCI and PontiacRegistry Paint and Bodywork tech advisor.
-r-