im throwing around a couple of different build ideas, has anybody ever used a b20 bottom end with a b18 head? i heard a few people say that the b20 cylinder walls were weaker, is that true?

The LS b18b head and the b20b heads are identical. Even the cams are the same. The only difference is the intake and exhaust manifolds, which is what differentiated the huge difference in torque curves between the b18b vs the b20b.
The B18B and B20B cams, valves, valvetrain are identical. Check the part numbers for all the parts at your favorite autoparts store and you will se that they are exactly the same.

thanx, learn some thing new everyday. i figured because the bore was bigger that the combustion chamber might be bigger.