decking will have an effect, but it will be really minor compartively (at least on a big displacement car, a small engine like in a motorcycle is a completely different story). gasket height is also important, but it's more an average thing that your average engine builder won't know beyond what the manufacturer says.
well stock with flattop pistons, the 289 came with a true compression ratio of 9.5:1. with the work you have had done, it sounds like your compression ratio is going to work out around 9.8:1.