Having 3/8" fuel line will not cause any issues. If you do a lot of constant full throttle, the 5/16" lines might. Normal driving, no problem.

The bushings won't cause any issues. If you replace the fuel lines in the future, replace the lines and sending unit with the appropriate 3/8" parts and just pull the bushings.

Having the mechanical and electrical fuel pumps together will not be a problem but be sure to regulate the pressure so as not to push the fuel needle off the seat. Quadrajets are easily flooded this way.

The fuel inlet on a Quadrajet has a separate filter holder. Using a pair of appropriately sized line wrenches, loosen the fuel line from the front of the carb. Pull it aside, then use another pair of wrenches to loosen the fuel filter housing/inlet. Be very careful not to cross-thread or overtighten this part, repairing it is no fun. You can add a fuel filter inline anywhere on the pressure side that is safe from moving parts and road rash.