350 or 305?

[fenderbluesman63]

i am having trouble identifying a motor i have in one of my parts trucks..the motor was represented to me as a 350, but i looked up the casting #on the heads and they are 305 heads..i cant see or find the casting #'s on the block (engine is on the 6-cyl mount position and pressed against the firewall) i wont be able to remove the engine for a while, so i was wondering if there is a way i can id the engine without the #'s..also the engine has a computer control distributor on it, but of course no computer...how badly is this affecting performance? the engine has a WILD cam, headers, edelbrock intake,carb, side pipes etc...it seems like it isnt perfoming like it should and i suspect the distributor isnt helping perfomance as it is sluggish...any help greatly appreciated...are there any external differences that would id this block like #of freeze plugs or anything? i am familiar with pontiac mostly and this is my first chevy, so please help the newbie!