Well if the woodwork is bad enough to be replaced and the car requires substantial welding, neither are going to be any good for datum points. So long as the car is supported in key areas to prevent too much movement, I'd remove all of the old wood replace with new then build to the new wood. Its easy to get a wood set square which then allows you to work on the steel. Unless its dramatically obvious that the steelwork is not square, i.e. B posts dropped or car collapsed between A post and rear arch I think this is the best approach. Its worked for us over the last 23 years.