Just my opinion: No header, only issue would be cold air on the valve guides. That might cause them to distort. Might. Back pressure? Not an issue. We are not talking about "high performance" here. Back pressure from impulse or shock waves? Come on we got around 3,000 RPM to work with. Look at "megaphone" type motorcycle exhaust systems for a reference. Not gonna be an issue here cause it needs to take the whole exhaust pipe length to matter. Exhaust manifold split works cause you shove all 6 into a bent and restricted cast iron pipe. Check it out, 2 90 degree bends plus a small dump for 2 ports and it gets worse for all the rest. Not a "back pressure" issue for sure. It might go a bit lean, better flow, but that would only happen after it started.
