I have found that the most accurate replacement trucks for Metroliners are very similar to those found on later RDC's. Athearn's RDC trucks look quite good on a few HO restorations I've done to a fleet of Bachmann cars that lost their trucks.
The extrusion has no window cut outs? Do you need to provide that yourself? If so how? Just curious. I have several Williams O "gauge" cars and would like to get a more scale Metroliner to play with.
However, Fred called it. The Williams late cars have the Genesis unit style side frames which are not very close. Also every single car in the set is set up to be a power unit and is weighted as such. A single powered unit will not pull the weight of the four car set up a grade. I added a second powered unit, but then robbed parts for a Genesis project instead ......
I would think that the Williams end caps might actually work in the extrusion you have if it is the same style extrusion. The cab side parts you got are definitely post Metroliner rebuilds into cab cars. Even after the Amtrak rebuild program, the Metroliners retained the single window for the cab. I remember them running in service between Philly and Harrisburg during their final years as powered units.