My $0.02... regardless of whether/or not the secondaries are opening, a 413 (or any other engine), wouldn't go just 40 mph, & flatten-out at 70mph! Sounds to me, that the timing is way too far retarded, and/or the distributor position is off. Disconnect coil wire, pull #1 spark plug, find TDC, on the compression stroke, and pull the distributor cap to see rotor position. Someone (?), may have installed the distributor incorrectly. The rotor on your engine turns counter-clockwise, and loosen the distributor hold-down bolt/clamp, just enough so you can turn it. Firing order is: 1-8-4-3-6-5-7-2. Turn distributor on your RB 413, "CLOCKWISE" - to advance the timing.
Pull the vacuum advance hose from the carburetor & put a screw in it to block it off (if one is connected to the vacuum advance). With a timing light on #1 plug wire, start the engine, and IIRC, it should be 10/12 degree BTDC, and that is where the timing light should pick-up the marks on the vibration damper. If it doesn't, find TDC, as with above, and pull distributor out enough so that when you re-install-it, the rotor, should be just "coming-up-on" the #1 terminal. That is your "initial" timing is. The vacuum advance makes-up the total. If the marks on the damper don't move as you increase engine rpm, then your distributor/vacuum advance is malfunctioning. You didn't state if you have points, or an HEI transistor ignition. If you have points (dual, or single), adjust to specs. REMEMBER...Timing does NOT affect dwell, but dwell does affect timing! If the stator, pick-up, etc. is bad in an electronic distributor, that is another possible problem.
Your cam has a dowel-pin, that aligns the cam sprocket, and the crank sprocket, with two small "0's" that should "mate" with each other. If the engine guy installed the cam "straight-up" (correctly), without a degree-wheel, and no "offset" cam bushing, then it's still possible the distributor is a "tooth" off. Or, your vacuum advance is stuck. If the engine turns over easily, after reaching operating temp, the timing is too far retarded. If it barely turns-over when hot, it is too far advanced, but that wouldn't make it run the way you said it does... Hope this helps, & GOOD-LUCK!