I could take offense at calling a 302 "wimpy", but I'll let it go since the guy saying it has 3 big block Cougars!!
My car is a 68', weighs 3200 lbs with me in it... do the 69/70's weigh that much more?? My best pass so far, with a wimpy 302, is 13.8...
There are so many variables to consider with his question... they boil down to how worn out is the 302, and how much pain and suffering to swap into a tired, junkyard 351W. Header locations are higher due to increased block height is one that just comes to the top of my head... Distributor is different... Hmm, what else...
People like Crane or Comp Cams can recommend a cam... how out of control do you want to be? is the question... I'd say you probably should stay in the hydraulic lifter range to avoid having to make frequent valve lash adjustments and having a noisy valve train... Plus a LOT bigger cam will get you into valve springs, full roller rockers, thread-in studs... etc... Lots of $$ in your heads.
You can make up a lot of compression due to your combustion chamber size... I think the 74's were big due to the emissions baloney... You'll know a lot more after you pull the heads, you'll see what pistons you have, see your combustion chambers, see you big giant EGR bumps in the exhaust ports, and you will be able to tell if you have a big ridge at the top of your cylinder bores from excessive wear. Take your time, drive a beater while the Cougar is torn down, and have fun with it...
Or just be like Royce, Big Power=Big Block!