I just went thru this when swapping out a 250 six for a 350 in an 82 C10. My biggest problem was determining which small block engine brackets (aka clam shells) to use. There are a bunch of them, and they all have slightly different dimensions.
I lucked out because the frame brackets are the same P/N for both engines, but the motor mounts are bolted higher on the brackets into a second set of holes.
Anyway, for anyone wanting to know what P/Ns were used in which year models, etc., this site was very helpful to me:
http://www.gmpartswiki.com/
To find engine mount bracket P/Ns you can do query on "bracket, engine mt" and enter the correct year, and you get a bunch of manuals and pages to choose from. Sometimes you'll get pictures, other times just the part numbers and descriptions.
BTW, GM calls the frame brackets "bracket, engine mt frm".