I have just built up a 383 with vortec heads and upgraded hei distributor with 50,000 v coil. Does any one know what plug and gap I should be using.

 

It depends on the compression ratio.The higher compression is, the colder the plug needs to be. So going by that, look at the recommended plug for an engine using those heads in stock form and usung Your compression, judge accordingly. Gap is usually safe in the .045" range.