It looks as if the engine mount reinforcing plates are spot welded too, Deejay. That means a spot welder is needed to do an authentic job.
By making the whole one piece deck then bolting it on, it is possible to straighten the deck precisely after completing it, before attaching it - to straighten the deck you don't need any equipment except a flat plate and some feeler gauges. With the side plate bolt-holes accurately located, the basic frame will then be square and precise - no welding distortion hassles. I think that will have been what the original design counted on.
