Almost all MTH cars have to be dismantled first to get to the truck screws. For flat cars this should be simply a case of unscrewing the screws that hold the underframe in place.
Once you lift out the underframe, the truck screws will be revealed. The thing is that Atlas trucks have a shoulder which comes up to meet the pivot point on the car and the screws attach from below. Conversely on the MTH cars (at least the modern Premier ones) the car underframe comes down to meet the truck and the screws go in from above. The two trucks are very different designs - when I offered them up I concluded there was no hope of making them work. I proceeded to obtain the appropriate MTH 2-rail trucks and fit those instead - for modern roller bearing trucks the part number is 20-89002.
With those the conversion is quite easy for most cars although I haven't tried the Centerbeam. Hope this helps.