Originally Posted by noelm
No, the "timbers" are called stringers. Each of the four stringers to which the engines mount ought to be capped in metal, either steel or aluminum, attached with both adhesive and bolted transversly through the stringers to spread the load and to spread the engine's thust as well. The metal capping the stringer is the engine bed.
OP, a whoole lot of labor is involved in swapping engines, if you are going to pay for the labor, which is the major expense in my experience, you ought to go with a more modern engine, like the QSM.