By now you probabily know that it is imperative to use the end brace on a GM starter. (along with the correct starter bolts as stated above) The two starter bolts alone are just not enough to support the heavy GM starter. Soon the bolt holes in the block fail from the starter jumping around. I'm suprised you were able weld up up the broken off corner of the block and redrill and tap the hole.
I tried to do this on a 396 but was never sucsessful.
Rebuilt starters should all come with a big red warning sticker and a new starter end brace included.
Once way to get around the hot start/slow cranking problem on a GM is to install a ignition interupt switch to disable the spark during cranking. Then just engage the spark and it will fire easily.