6.2 diesel manifolds are exclusive,I don't think any gas engine manifold will bolt up to a 6.2 head...but if your putting a 6.2 in place of a gas V8 ,the gas manifolds will end up in the same spot as the diesel's,so your old exhaust pipes should still bolt right up...far as I know the 3 bolt pattern on the flanges are the same on both..
I wish I could remember what number starter I sold those guys years ago--if I remember correctly it had a cast iron nose,not aluminum,and it wasn't used on too many vehicles..I think it fit only engines with the larger 168 tooth flywheel..
As for the engine block not having a tapped hole for the front brace,it should still have the "boss" casted into the block,that can be drilled and tapped for a bolt..
I know a guy who welded a 6.2 block that had a big chunk busted out where the outermost starter bolt went,and it was a success..he used nickel rod with a DC arc welder...but he's a certified welder too,and can weld a lot of "impossible" things and has good luck too..
I had a "free" 350 given to me that had the block snapped at that starter bolt location years ago--a junkyard had it,when they pulled the engine from the vehicle,it swung on the chain wrapped around the loader's forks,and it crashed to the concrete floor..they just heaved it in the scrap pile!...
I had a truck with a SM465 and a cast iron bell housing,that had a starter with 3 bolts that bolted to the bellhousing,not the engine,so I was able to make use of it...that engine ran nice too!..but was only "good" for that one particular application..