225 slant six vs 318?

I had a slant 6 in my 68 Barracuda for four years. It had the head decked for 9:1 compression, Comp 252S cam, Offy 4bbl intake with a Holley 390cfm 4bl, Hooker 6 into 2 headers with dual exhaust. This ran through an A833OD 4spd and a 3.55 geared 8.25 sure grip rear end. The car ran well was fun to drive but was far from what any one would call "fast". Never had it on the track but it was no where near as fast as my Shelby Daytona that ran 15.2's and was slightly faster than the Avenger my wife had that ran a best of 16.9. I would estimate this car ran low to mid 16's.

Some head work; machining for for bigger valves, basic porting (bowl blend and gasket match) would likely have gotten my car into the mid 15's, more cam low 15's.

A low 14 high 13 second slant is has gotten into the range where you are making compormises in driveability, it's going to need more gear, looser convertor if it's an auto, etc. There are 10 second slants out there but they certainly are something you could use as a driver even if they are road legal.

Like the other said make a plan. I did and stick to it. You certainly can build a slant that is fun to drive without spending a bundle and if it looks like mine it will gather a lot of attention. But if you want to go fast a v8 swap to the 340/360 will be more cost effective.