You could glue pins of the appropriate diameter in the spindles/ suspension to replace the metal axle. Use styrene rod, wire, or cut the kit axles. Use the appropriate glue. If you cut the kit axle or hardened wire wear eye protection and make sure you use cutter specifically for hardened wire.
Not all versions of the AMT '49 Mercury have flathead speed parts. I have several of them with just a stock flathead or the 413 cross ram Dodge (I know it's a MOPAR anyway) as the only options.