While I don't know all the tech specs, I would say it is plausible. As a matter of fact I'll go out on a limb and say that Illumina has done it before.(Hopefully the limb doesn't brake) The 4.5/4.9 were basically the same except that the 4.9 got more horsepower and stronger internal parts.
The 4.5 and 4.9 I believe had the same heads and block just the 4.9 had different crank, rods and or pistons.
Think of it like 350 Chevy vs 327 Chevy.
Only drawback is that the 4.9 revs even worse than a 4.5.
As for converting a 4.5 car to a 4.9 it should not be that hard, I will almost bet you can cheat the system and use the stock 4.5 injection system on the 4.9 it won't be optimized for the extra .4 liter motor but should still run just fin as a cruiser motor.