Exhaust Question

2 1/4 should be adequate for your 318, but as abodyjoe says, why not just go ahead at this point and install 2 1/2 and be ready for possible future changes. Another item is, if you are looking for J-heads for that stock bottom 318, you would be way better off with a set of 302 heads or magnum heads, as far as performance. JMO, Mike