By all means I wasn't knockin the r block. Its a great block. Its just that boss is said to be a way better replacement.
But there is one advantage the boss has over your R block that your engine builder can't do. And that's the make up of the block material. The boss is said to have diesel block grade iron plus better cooling than the r block. But like you said any good machine shop knows how to fix that in the r block which it sounds like your guy has done.