Fuel pressure.
 

By the time fuel reaches the carb. , is the pressure the same as it was when it left

a mechanical pump ?

Are you talking about the fuel just reaching the carb , or like in the case of the Q-jet the fuel first passing thru the filter ( hopefully paper) and then into the carb?

Fuel pressure after the filter
 

After the paper filter.

What Carb and what pump are you running, and what fuel pressure is your chosen pump rated at, and also how much ball park HP will this motor be making?

simple answer... yes. the short piece of line from the pump to the carb doesnt loose any or very very little psi. & an unrestricted paper filter shouldnt lower it much if any either.

are you having a fuel psi related issue?

Will depend on flow rate. At low flow rates (idle) it should be exactly the same. At high flow rates (WOT) it will be less, and depend on how much restriction.
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Yes, Pascal's Law.
https://en.wikipedia.org/wiki/Pascal%27s_law