Can you run a fuel cell but still use a mechanical pump on the engine instead of an electric pump in the rear? How well will this work?

Yes you can, will work just fine.

have a 16 gallon cell with a carter mechanical
pump, works fine

Cool. I'm glad to hear that. I think my stock gas tank has alot of sludge in it. It seems the engine is starving for fuel and the fuel pump seems to be pumping just fine. I figured it's time for a new fuel tank.

I'll be using a fuel cell with a Carter 172 with 1/2" line front to back (thanks Tom B.) myself and am glad to hear that setup will work "just fine" :D

Hey....just got to thinking...does anyone make a replacement fuel tank to go in the original fuel tank location but still has the bottom sumps like a fuel cell has?