I haven't noticed anyone posting a reply to your question, so I'll give it a try. Iskenderian (Isky Racing Cams,
www.iskycams.com) makes adjustable guide plate pairs which get tack welded after alignment and adjustment. These are part number 300AGP for the 3/8" pushrod size. Comp Cams (www.compcams.com) make something close for the 455 Olds. This is their part number 4843-8. Of course the above part numbers are packaged for 8 cylinder cars so you will need to buy 3 sets to get 4. The Olds valve to valve distance is about .030" larger than the 292 Chevy which means that the valve ends of the rockers are about .045 " closer together. Just make sure that the roller (or whatever} is still bearing properly on the end of the valve stem. Precision Engine Service 2511 Providence Road, South in Waxhaw, NC 28173 also makes guideplates for the late Chevy 6.
Hope some of this is info that you did not already have.
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Ron 1801