on a 400 big block with a stock mechanical pump what should the fuel pressure be around..i am showing on my gauge around 3-4 lbs .. i am thinking the mechanical pump is the reason why my motor want stay running , i put a holley 4 bl 670 street avenger.. not stock motor was rebuilt about 3 notches above stock on the cam .it seams it is staving for gas when i give it gas it is like sucking air. any help would be much appreciated..thanks

Check your fuel pump pushrod to see if it is worn down it should measure 3 1/4 inches. You should be running around 6 psi.

I normally run my 440 at 4 psi, that should not be a problem as long as there is the volume you need at higher RPMs to maintain 4psi.
Start with the simple things,
new filter, check the lines for suction leaks, float level in the carb and the fuel pump pushrod is becoming a bigger problem as oil quality goes South,(lack of ZDDP)
Might have to pull the fuel sender out and replace the filter sock.., but normally the carb will contain enough fuel to get things rolling and fuel starvation only occurs with the carb empties out..