i have a board mbs channel trucks and want to use them on another board, but the top part of the trucks are welded onto the steel frame of the board. will the bottom parts of the trucks fit onto the newer top parts. the mew ones look like they have the same design with just a new material.

the older board that i have is the mbs reactor

You're going to have to sit down and have a long talk with yourself (preferablly someplace private so people don't think you have gone off the deep end :o ).

If these are the trucks, they already have a fair number of holes (weight-shaving) on the top hanger plate. To then add additional Matrix compatible holes is going to be dicey. If you try to use the existing holes and somehow retrofit it to the board, whenever you go to upgrade the trucks you are going to have to redrill whatever you decide to use for the deck.

This is after to either break the weld or saw and grind the top hanger plate free.

If you are bound and determined to do this, I would contact MBS tomorrow and tell them what you have. Ask them if you can reuse the tires/axle, lower hanger, springs and elastomers. See if they can tell you if this setup would be compatible with either the Matrix or Matrix Pro top truck/upper hanger. Depending on what they tell you you might have to do some swapping of kingpins/bushings.

In the end you will end up with a kind of weird hybrid. If somewhere down the line, you ding/bend any of the axles, you will be replacing the lower portion of the trucks, since that and the axles are all one piece.

This isn't really a matter of cross compatibility as much as compatibility between two totally different generations of a product line.

If you go back 4-6 months (and even further back) in the For Sale section, there are a fair number of boards/decks in various states of completeness that could get you started on kitting out a new board.

ATB,
Sam