If you are talking about a GM 454 it should have a Turbo 400 transmission. The most direct replacement would be a 4L80E. It is a 4 speed automatic with 4th being overdrive and it has a lockup torque converter. Both should help with fuel mileage. It is comparable to the TH400 in strength, as many of the internals came from that transmission. You will have to have a computer to run it, driveshaft and mount will need modified also. PM me if you need more information on availability of the parts.