Bob is right, the 4 tangs on the stock motor mount still hold on the middle of the body in my setup. But they engage with the plastic floor piece not the hood piece so once you have the loco weighted like this, do not try to pick it up by just the hood... 
Also the couplers still hold on the front and rear as long as you use the Red Caboose coupler boxes. Might need a small shim to get the coupler height dead-on the Kadee gauge depending which trucks you use e.g Weaver plastic, P&D GP or F Unit. I think there are actually 3 bolster height variants of the P&D brass trucks and only one gives the prototypical height for the GP9. I confess I'm not sure which one. Bob would know for sure! 
