I have Edelbrock headers and they came with spacers and longer bolts, so everything fit without modification. The spacers are about an inch long, and basically just pieces of pipe. They take up the slack that is there due to the thinner flanges of the headers as compared to stock cast manifolds. So, on that theory, you could measure the thickness of the original manifolds and the thickness of the header flanges, subtract one from the other and then just make a spacer that length out of some tubing with an ID slightly larger than the bolt shaft you are using. Just buy a 4 inch long pipe nipple and cut your spacers out of the unthreaded portion of that to make the necessary spacers. An angle grinder with a cutoff wheel makes quick work of it, but a chop saw makes a cleaner cut. If you want, I will take a picture and post it. Just let me know here if you need it.
Oh, one more suggestion: buy lock washers for the header bolts. Mine came with them, and I never retorqued them in 45k miles, nor did they ever blow a header gasket. Also, if you can run them, the Mr Gasket round port gaskets in their Ultra-Seal line are almost bulletproof. They have only a 1.5 in diameter, so if you are using large port headers you may not want to use them. I am not, and they were similar enough to the original gaskets that I used them when I put my engine back together last year. (Dropped a washer down the intake, and started it....ouch!)