87MagnumII

I'm wanting to take my 350 out and make it a 383. Some of you remember my last build from two years ago and I really appreciate all the help then, but I have the itch for more power and want to change a few things and fix a few things that I've figured out from the first build

Say for instance I want to run 15w40 over 10w30 in this new engine, how does one properly "tolerance" the main and rod bearings for the heavier oil? Is there a "magic number" for SBC tolerances for different weight oils? Does machine work need to be done or do they make special bearings for different applications? My engine builder did it last time and set it up for 10w30. (when I say engine builder I mean my family owns a small machine shop, and this guy does mostly OE type work but has dabbled in some higher power stuff, and my labor is free since I'm a broke ass). I'm not looking for a big $$$$ engine. Just something reliable and has good power. Would 10w30 be ok or do I need to get more in depth this next time around.

Please don't turn this into a "you need a marine builder thread". That is not an option for me at the moment so let's play the hand that is dealt here and work with what we have. He is a good builder. Just limited marine knowledge and we are learning together as we go.