Ref points are great for "coarse" planning but you should not install a motor and trans into it unless you mock it all up, settle everything on it's purposed place. Check carb mounting surface for level, check all kinds of clearances like steering box/column shaft, exhaust, hole in floor, dizzy, fan, pulley, pinion angle... before drilling or welding. Not to say you are doing this but you shouldn't do it by measuring as others have said.
