Re: hard starting after setting for 8+ hours
Your parts are very much like mine, the small block and 1406 carb and Performer intake. My intake crossover is blocked to reduce exhaust heat in the intake.
The Edelbrock 1406 Carburetor guide has a section for troubleshooting on pages 34-35, though it lists ignition timing, fuel filter, and underhood temperatures as causes for hard starting. One indication of trouble is the lack of fuel in the carb bowl for the accelerator pump to prime a cold engine. I would recommend to remove the primary rods and jets and check they are clean and the rods move freely.
I believe it is normal for the fuel level in the external fuel filter to drop a bit as the engine cools. Mine does the same.
Fuel pressure of 6.5 PSI is correct. Residual pressure hours after shutoff means the fuel pump does not leak.
I suggest you retighten the intake manifold bolts to verify the new gaskets are sealed. Torque the bolts in sequence (figure 2 in Edelbrock 2101 instructions, a clockwise spiral from center to the edges) to 25 ftlbs.
Next I would say to connect a vacuum gauge to manifold vacuum to check for a leak in the new intake gaskets. A mild cam should pull about 15 to 18 inches at idle. A lower vacuum reading at idle or a fluctuating needle would indicate an intake manifold leak or leaking intake valves.
Hope this helps you.
1970 Chevelle Malibu purchased new in 1970
355 SBC, XE262H cam, Performer RPM heads, MSD HEI, TH200-4R trans, 3.55:1 Posi, digital gauges, QA1 coilovers