Headers for 67 barracuda 318 with no modifications

Why bother,
AFAIK there are only two good reasons for to run a header, namely;
Scavenging and Inertial tuning; neither of which are desirable on a stock 318.

The stock cam has like 20* of overlap, so scavenging is not gonna occur.
Inertial tuning sure but to what effect?
You are likely running gears in the window of 2.76 to 3.23s, Your rpm is gonna be in the basement about 80% of the time., and the engine heavily throttled, perhaps 95% of the time. Headers for you, are more or less dead weight and a waste of money.

Buy a torque-convertor instead.
Money way better spent.
I like a 2800.