I've just done a neck reset on a '58 D-28 and it looks like the bridge is still original with the through saddle. I will be reglueing the original bridge back on( it was lifting ), and replacing the saddle (the original was notched deeply to lower the strings). The bridge thickness at the E1st. pin hole is 5/16" and at the E6th pin hole is 11/32". Hope this helps.
I can't comment on the modified thru saddle.
the bridge question is a little dicey, but i am going to give my opinion. Please do not build a drop-in through saddle for that guitar. you need to make the bridge look exactly like the original, you can move the saddle location if needed, if the saddle is in the wrong place. it should be pretty close from that era. you will be helping the guitar retain value. you need to do these repairs with historical value in mind. although, there is nothing wrong with your idea, you just don't want to do that on a vintage guitar.
andrew, you can disagree, but Dave, i strongly suggest you build the bridge the way it was made originally. i will not comment any further. if you need further advice i would call Gruhn, Retrofret, Matt Umanov, Gryphon, anyone else heavily involved in the repair and sale of vintage guitars and ask their opinion and i think your answer will be clear.
I've seen both types of saddle support fail and there are a lot of thru saddle bridges out there that work just fine. I realize that it's much harder to repair but you're making a new bridge so that's not an issue. The repair you are doing should last for decades and since this is on a middle vintage Martin which should have room to appreciate over the life of the new bridge, I would stick with the original design.