I have a 1974 chevrolet monte carlo with a 350, recently a replace the fuel pump (still mechanical) and drives great, no stumbles, easy start up, etc.

My confusion is that after I drive it for a while I see that there's no fuel on the filter (glass see through inline) and 0 psi BUT the car keeps working just fine, after letting it sit for a while I start seeing the fuel but idk, I checked the carb (edelbrock 1806) and bowls are full so I suppose is for that, but still kinda confused if that's ok or not.

 

The 74 has a fuel return line emissions and taking it easy on the float needles to make it less likely to flood allow it to run low pressure. Pressure sounds bad you should have some not zero ,there is a restriction some where . I've had new hose defect on the inside do the same thing. Theres no way a single float needle can out demand "a good pump" some where it's getting choked maybe clogged or kinked line . Never bad to put new hoses everywhere. If you've never cleaned the tank and or replaced the pickup filter I'd go there last its heavy , messy and lots of dangerous fuel related work .