Headers or not...
Both my 73 BetaCat and the wife's 70 Mach have 351C-4Vs and neither has headers. Wife's Mach is a 21,000 mile all-original, untouched, unrestored and will never see headers! It runs just fine with the 4V exhaust manifolds and full duals! BetaCat also has the stock 4V manifolds and full duals, but 'turbo' mufflers instead of the stock mufflers. Wife's Mach will turn a 15.0 with the stock exhaust, and would go quicker if she'd let me install a posi rear end! BetaCat will run 14.40s with just a rear gear change from 3.25s to 3.70s, but you also have to remember that CJ cars got a factory 10.25" high-stall converter! BetaCat is getting a little winded above 5200rpm, but that is due to the camshaft more than anything else.
You have to remember that if you're changing from 2V to 4V heads, you'll aso need to upgrade both your intake manifold and your exhaust manifolds to 4V port sizes! Remember that the 4V heads have bigger intake and exhaust ports than the 2V heads! The biggest problem you'll face is finding a set of 4V exhaust manifolds. And to think of the sets that I had thrown away back when...
Another thing to remember. If you decide to go with headers (if you do) also make sure to specify 4V ports! You can generally use 4V headers on 2V heads, but not the other way around. The ports on the 4V heads are so much bigger than the 2Vs that the sealing bead on the header flange of 2V headers won't cover the 4V port!