One note on the stepped washers. If you use the ceramic coated 6 to 1 headers from MSA the stock stepped washers for a 240z should work just fine. I don't know if the flange thickness on the stock 280 exhaust manifold is the same thickness as on the 240 so this is also a consideration on the stepped washers. I did have to do a slight grind job on the thermostat housing to allow enough clearance for the MSA header flange. MSA also says you should use their header specific intake/exhaust gasket with their headers. Don't know if the thermostat housing interference I had on my 240z would also be true with a 280z thermostat housing.

MSA 6 to 1 Ceramic Header Problem - Exhaust - Classic Zcar Community

@Zed Head and @w3wilkes thanks for the heads up on these issues. We will need all the guidance we can get going forward! lol