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An external pump like a GSL-392 is fine. It flows a lot more than a stock 5.0 HO needs, but it should work as long as it is functioning properly. Fuel pressure at idle should be 39psi (2.6-2.7bar?) with the pressure regulator vacuum line disconnected and plugged. Fuel pressure should go down quite a bit when the vacuum line is reconnected, perhaps to 25-28psi.

Your engine appears to be from a '94-95 Mustang GT, but the upper intake is from an '86-93 5.0 HO. That will render the EGR inoperable and may trigger a code, but is not causing the cold start problem at hand. Make sure the coolant and intake air temperature sensors are reading correctly and that the TPS is set and working properly.
 
I never thought that its possible to have a good running engine without ALL vacuum hoses. So happy it will work with the most important ones! Im going to check prices on rockauto and LMR.
Also i want to replace the complete engine gasket and clean it up and maybe its time for a new fuel pump after 10 years of working for my Mustang

It will take some time but when everything works i will leave a message here.

Hope you all get well through this Covid Time and thanks again for youre help!
 
Well you could show us what you get, tell us were you got it and what it takes to get parts across the big river, there has got to be more than one fox car in Germany, you could be helping out a brother or sister over there, a lot of stuff ends up googlefued threads from here show up ALOT, besides, you just may become famous over there, like me, I'm famous over here, or is that infamous? I forget.
 
I never thought that its possible to have a good running engine without ALL vacuum hoses. So happy it will work with the most important ones! Im going to check prices on rockauto and LMR.
Also i want to replace the complete engine gasket and clean it up and maybe its time for a new fuel pump after 10 years of working for my Mustang

It will take some time but when everything works i will leave a message here.

Hope you all get well through this Covid Time and thanks again for youre help!

Dont just assume its the pump...test the pressure at the rail with a fuel pressure tester first. The pump may well be fine, but in-line pumps do tend to burn out faster than in-tank pumps simply because they get hotter since they aren't submerged in fuel. I always like to see a full 43psi at the rail without a vacuum line connected to the regulator...not sure what the 5.0L ECU expects for pressure though.
 
39.15psi is the exact spec for '86-95 5.0s and a lot of other EFI Fords from that era.

The fuel pressure needs to be verified anyway, but I disagree that external pumps are more prone to failure from less cooling. All the current-handling parts of the pump are flooded with fast-moving fuel when the pump is on. It can really only run out of cooling due to flow restriction or going dry. An in-tank pump might not overheat as quickly from dead-heading, but it's still relying solely on thermal conduction through the steel case and magnets which I don't think amounts to much.