Deejay, it looks to me as if that deck has considerably deeper flanges than the twin rail, and it is also double-thickness on the horizontal deck part. You can see the arc welds at the side: two L-shaped reinforcements are welded underneath the flat top deck, and the heavy engine mount reinforcements appear to be spot welded to the L-shaped pieces. Comparing that with the twin-rail deck is like comparing a piece of heavy steel plate with a piece of steel sheet. The deep flanges (both around the perimeter and across the middle in two places) give the deck bending strength, and the double thickness deck plus engine mount reinforcements give cracks no opportunity to start at the engine mounts. The engineer who designed the twin rail deck obviously thought the engineer who designed the 45 was a dill.
It also seems to me that the best thing to do with a cracked twin rail SB45, is to fabricate a single rail deck to the original design, and bolt it to the side plates rather than weld it.
