new to me 68 Barracuda Conv, sluggish 318

As others have stated, it could be a low compression 318. Putting performance heads (if they are truly are J heads) won’t help unless it’s in combination with other items. It could be a smogged distributor with a super slow/long advance. Timing could be way off. Transmission might be slipping too. Rear end gear might be low (2.45 or 2.76). To boil it down, it could be a bunch of things. Does the AC work?

Enjoy the new ride!