Ive seen several dyno tests on mild 318's with headers, 20 hp is probably the most you will gain with headers on a mild 318 which is barely enough for the seat of the pants feel and only 2/10 in the quarter
as yy1 said is the headache, price, and frig factor worth it
a set that is a great fit on one car can be a nightmare on another even a 1/4 inch in difference can add a huge frig factor
340 manifolds give about 7-11 hp more than standard manifolds
http://www.moparts.org/Tech/Archive/exhaust/9.html
if you absolutely have to have that great crisp headers sound then go for it!!!!!!