a turbo 2.3 and a 5.0 are going to net you around the same mileage ( i've had both in mustangs)
if you want to stay with the 4 banger just swap a complete turbo motor and get a tune make sure you run an intercooler either the top mount with a scoop or front mount or your really limmited to 5 or 10 psi.
mr gasket makes a nice mls hg for the 2.3 and arp makes studs jegs had the stuff in stock when I built my car
if you turbo a non turbo block your just lighting the fuse on a time bomb