According to these guys - https://www.hotrod.com/articles/testing-mopar-273-power-pak-dyno/ - TTI headers were worth 11 peak horsepower over the stock manifolds. That's with the stock hi-po cam. They didn't test with any other cam, and also don't tell us how much the low and mid-range picked up. Often, the increase in that area is more noticeable than the increase in peak power.
They do show you how they modded the TTI exhaust flanges for the 273 heads. (I'm not sure what your 318 police heads are; if they are basically 360 heads I assume the TTI's would not need to be modified in your case.)
Besides the headers, you'll also need (according to TTI) a special starter that costs somewhere in the neighborhood of $300.
If you're running a 904 trans, your installation would probably be easier than my 4-speed with 10.5 inch 340 bellhousing.
With reasonable mechanical ability, I'm sure you could handle putting TTI's in a 65 Barracuda. Or Doug's. I reckon only you can decide if the power gains would be worth the trouble.
