if the trannys are both automatic, then you could run into a problem with flexplate matching to torque converter... specially if they aren't the same auto.... i ran into this problem in a similar situation, but mine was because i was using a c4 sized torque converter on an FMX but needed the flexplate still the larger FMX size so the starter would match up... ended up having to drill the flexplate to fit the smaller torque converter.... as long as you dont do any monkey buisness like this you should be fine... even though a smaller torque converter is an advantage, will lighten the load on your engine and can give U a lil extra acceleration on that heavy vehicle