Fuel level sending unit

I just swapped my stock mach 1 tank over to a 03/04 cobra tank from On3performance. I used all the original parts from the Mach 1 like the hat and sending unit. I got everything back together and put the tank back in and went to start the car and it started right up. Went to go fill the car up and the gauge won’t show that it’s full it shows a quarter tank(where it was before the swap). The gauge isn’t stuck because it goes down when I shut the car off. What I’m asking is does the sending unit from the Mach get stuck on the baffle and if so does the 03/04 cobra sending unit bolt onto my 2003 mach 1 assembly that holds the pump? Will Appreciate any info
 
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This is IMO. When I looked at RockAuto and compare the fuel pump complete units between the various available engines, the float appears to be the same. Further many of the actual part numbers are the same. That tells me that any differences in the internal gas tank baffle design does not require an application specific fuel pump basket.

Since I'm a big believer in Occam's Razor, to me this says that your problem is more likely:
  • a bad sending unit itself
  • related to an installation error. For example incorrect indexing or damage done during installation.
Just my 0.02 worth. Sorry.
 
This is IMO. When I looked at RockAuto and compare the fuel pump complete units between the various available engines, the float appears to be the same. Further many of the actual part numbers are the same. That tells me that any differences in the internal gas tank baffle design does not require an application specific fuel pump basket.

Since I'm a big believer in Occam's Razor, to me this says that your problem is more likely:
  • a bad sending unit itself
  • related to an installation error. For example incorrect indexing or damage done during installation.
Just my 0.02 worth. Sorry.
Honestly i thought I miss have got the arm for the sending unit stuck on one of the baffles. To me it seems as though the sending unit arm with float on the 03/04 cobra is bent in a way that makes it angled so it doesn’t hit the baffle. I’m just wondering if the 03/04 cobra sending unit will bolt onto the hat/fuel pump carriage from my mach 1
 
Check it out for yourself. Go to RockAuto.com and compare the part numbers. Look at the pictures. Draw your own conclustions.

There is also the issue of "indexing". I didn't do the swap so I don't have the experience of seeing the parts. I have dropped a few Mustang GT's in my day. But no Mach 1's or Cobra's.