Surging while cruising at steady speed. Idle and acceleration great, Help!

Well I finally got my Dart up and running with a brand new 408 that I had dyno'd. I got it aligned and took it for a shakedown run and I've discovered that I have a surging issue while cruising. In fact any time I try to maintain a steady rpm (ex. if I push the clutch in and try to hold the rpm at say 2k or 3k rpm it surges and sounds like its missing). If I give it a tiny bit of gas to accelerate it smooths out immediately and accelerates nicely. Seems to idle great and it also has super quick throttle response at any rpm. WOT acceleration is right on the money too. Starts up when hot no problem and when its cold you almost don't even hear the starter go it starts so fast.

Here's what's on it:
408 Stroker, Edelbrock heads, Quick Fuel 750SS (the engine builders that dyno'd it jetted it at 68 and 74. 3/8 fuel line, Edelbrock 110 GPH fuel pump at 6.5 psi.

Timing was set at 19 initial and 34 overall. It's running an MSD billet distributor (no vacuum advance), MSD coil, MSD wires, plugs gapped at .045, and an MSD 6AL2.

My engine builder suggested it was probably running lean because I'm running open headers so they suggested I jet it up a bit. I installed 74 primaries and 78 secondaries. Problem almost seemed worse so I took out the 74's and installed 72's. I also double checked the floats and made sure they were set correctly. The problem is still just as bad. I have no other jets so I can't jet it down to 70 unfortunately.

I completely took for granted to check the timing that they did so I plan to do that tomorrow morning. Any other suggestions?