I think I replied to your question on another forum but I can't be sure, so here goes.
I have a 73 convertible with 70K miles and the same Q-code (351 4V) motor with a C6 trans and 3.25 gears. I have been messing with it for the last 15 years trying various things WITHOUT getting into the long block. Basically it still has the factory cam, heads, short block and exhaust manifolds. I have added the Edebrock Performer intake with a 600 cfm Edelbrock carb, 3.89 rear gears and a 2800 stall converter. I have done a LOT of research as to the best way to go with the motor and have decided on a plan of action.
First of all, you have the "D3" 4V heads which are the worst of both worlds. They have the big 4V ports with open chambers and the smaller 2V valves. You also have dished pistons with a very low compression ratio and a 4 degrees retarded timing gear set. The 4V heads are just not going to make a lot of power below 2500 rpm. That's just how it is. So the first thing you have to decide is if you want to keep the 4V heads or not. If you want to stay 4V you are going to have to come to the understanding that you are going to have to rev the motor in order to make power and you must build your combo accordingly. I'm talking 6000 rpm or more. This means steeper rear gears (at least 3.70) and a looser converter (you have the CJ converter which stalls at about 2200 but believe me going to a 2800 stall converter makes a world of difference). You need a more aggressive cam, a better intake (the Performer intake is good to 5500 and that's it) and a bigger carb. You also need headers and a solid valve train. Replacing the factory valves is a MUST - they have a nasty habit of coming apart which is very bad for your pistons.
If you try to keep your heads I would recommend that you put the larger 4V valves in them. Clevelands like compression and thus your idea of popup pistons seems like a good idea but only within reason (like 9.5 or lower). The open chamber heads have a much greater tendency to detonate (ping, knock, etc.) than the closed chambers do.
Here's what I'm doing. I bought a set of "done" closed chamber 4V heads off ebay. Screw-in studs, valve guides, one-piece stainless valves, decent springs. I got a 218/222 cam with .510 lift on both the intake and exhaust. Roller rockers. FPA headers (they fit the best though pricey). M-code (squarebore) cast iron intake (just like a Boss 351 intake, just heavier and a LOT cheaper). 2 1/2" exhaust. Carb still up in the air although my Boss 351 really liked a 650 DP. I'm planning on adding all this to my stock short block and see what happens. Even with closed-chambered heads my compression is not going to be that high (probably low 9s). This combo should pull hard to 6000 but still be very streetable with a decent idle. I figure the stock short block can handle it.
If you are on a budget, watch ebay for a set of heads (OC or CC) that somebody has already done. The guy I got mine from was running low 13s and decided he wanted more power. He bought a really worked set of aluminum heads and was getting rid of these. Luckily he was within 100 miles so I did not have to pay shipping. He easily had $800 worth of parts and labor in the heads and I got them for $650. An M-code intake is good to 6000 rpm and won't break the bank. The Comp Cams XE262H is a great cam for a streetable Cleveland. You should easily be able to get into the 13s with that combo.
If you decide you want more power you are going to have to build the motor to rev higher and that means the short block must be stout. More aggressive cam and some porting will be necessary.
The Edelbrock heads are decent but they have 2V ports so you will be building a 2V motor. Not necessarily a bad thing because it will have more low end power and will still rev pretty well. Just doesn't have the high end potential of a 4V.
Good luck.
