I have a 283cc in a pickup with 40000 km on it but it has been modified from origional. it has a block vin from 1962-1964 but looks like 1980's top end and has an HEI distributor. how do I dertermine the spark plugs and gap for this configuration. The vacuum gauge is showing late timing, is this due to my gap? Does not seem to have much balls but it starts and runs o.k.