We’ve talked about the MPG possibilities of Crown Vics on this site in the past.
If you are getting 28+ MPG on your Crown Vic you are the boss Crown Vic MPG guy.
I think that with the mods you are proposing (including the lightweight engine) 30 MPG in mixed driving should be do-able. Add in the T-56 and 32-33 should be in order.
One guy wondered if you could get 40 out of a Crown Vic. My take is no – unless you go to a diesel engine. The diesel gives you 25-50% better efficiency although it weighs quite bit more.
Here is a youtube on a guy who put a Cummins 4BT3.9 into a Crown Vic. He claims 33 MPG mixed and 40 MPG highway using the stock automatic.
I haven’t been able to find any more on this car, but it looks like a good job. Sure hope he put lots of sound deadener in it. The Cummins is noisy.
Other options might be a M-B 617. Super reliable but underpowered and mating the engine to an American drive train requires you know a thing or two about DIN bellhousings. I don’t.
Another possibility is a VW TDI turned way up, but you are back to the weird DIN transmission connection.
I don’t think a GM 6.2 can be stuffed into a Ford engine compartment. A 6.2 or 6.5 will barely fit in a GM “B” body but forget the turbo.
The 4.5 Powerstroke (from a Ford LCF truck) might fit and it is just a 6.0 Powerstroke with two cylinders missing but it is a bear to find and a worse bear to source parts for.
Oriental fork truck and generator diesels do not have injection systems set up for road vehicles.