You not only need bigger heads with the 347, you want a good intake (better than your Cobra) and more cam than you think you need.
Since you are asking these questions, you might ought to consult a good shop that's done this 100 times already. They can steer you in a better direction. It'd be a shame for you to fork out all the money it will take to do a good 347 and then have it make like 350 HP.
'88 GT hatch. Full Maximum Motorsports suspension including bracing, race springs, Tokico Illumina struts and shocks. '87 Turbo Coupe rear end with 3.55 gears and disc brakes. Lincoln Mark VII calipers, '86 SVO master cylinder. BBK CAI, BBK shorties, MorFlow catted H-Pipe, Bassani cat-back. Steeda shifter. LMR GT500-style chrome 17x9 wheels, with 235s in front and 245s in back. Full Dynomat interior insulation. Blaupunkt head unit and speakers. Medium Regatta Blue paint with Titanium GT trim.