That spacer is for the 4mm crank, not the long rods. Measure the stroke to see if its a stock stroke or 4 mill (54mm stock, 58mm 4mill). If you go with a long rod crank you'll need to run 795 series pistons instead of the normal 513's. If its a hot rods crank it'll have a number in the side of the rod that will tell you if its a long rod or normal.
The spacer plate method sucks though. Id have the jugs ported for the 4 mill crank and buy 4 mill domes.