This is a good addition to the circuit.
I see that you have added R7-C8 to delay the pulse detect into the reset input of the first counter. I was wondering about that. Now as the first pulse comes into the clock of U3 the reset will be sure to be low so the counter will start. This was an ambiguous state before without the delay.
I think there may be a problem with the input circuitry. Either the voltage from the turn signal switch is there or it is an open circuit. It does not go to ground. There is no way for C4 to discharge except through the input impedance of U2.
Still trying to make sense of the D2 U4 combination. Is the polarity wrong on C9?
That Federal regulation must be for newer cars. On the older non-computer cars, the turn signal flashes slower when a bulb is burned out, as it is a thermal device. Less current through the flasher longer to heat up.
This is an old car built before the “We know what’s good for you” intrusive do gooder standards. Small real chrome bumpers, no computer controls big brother OnStar monitoring system, etc. I can’t see where adding DOT approved lights from another vehicle is going to be a legal problem, especially as there will be three times as much light emitted.
We did add disc brakes and three point seat belt shoulder harnesses. Also electronic ignition, sway bars, and radial tires. The car handles better than when it was new, puts out less smog and stops better. If you count all the materials used and processed, the energy needed to transport the material, run the factory, etc. it is ecologically sounder to fix up an old car than make a new one.