as for the engine it might be your timing, its really hard to tell whats going on without hearing it in person. mhh thats really werid that it stumbles at such high rpms but not a lower ones maybe your not getting enough fuel at hig rpms. as for the shock plate try Year One....they may be able to help you out...GM really doesnt offer many replacement parts for older cars. you have to go to a place like Year One for stuff like that