Do NOT use washers. Any kind! Not for a street driven daily driver. Go with the original type intake/exhaust clamps or have a welding shop custom cut some pieces of steel or irona and grind them to fit with your grinder. Be sure to use case hardened or grade 8 Bolts to secure everything down.
I pulled a set of clamps off an old 235 block and they work perfectly on my Offy single 4 barrel intake and Dynomax tube headers.
The headers came with their own bolts and washers but they were too short to use with the OEM clamps. I used a couple of the on the ends to secure the exhaust. But for the shared mounts I used clamps and a combination of hardened bolts I had lying around that would fit.
I experimented with several things before deciding that washers and short bolts just won't do. I bought stainless exhaust header bolts from Alloyboltz. Didn't work. Here's what I found when I used washers and short bolts.
A. They allow header movement, even when torque beyond recommendation.
B. They never fully sealed the exhaust headers to the head.
C. Non-case hardened washers will certainly bend over time and cause leakage.
D. Case hardened washers will still allow the flange to slip and move underneath, no matter how tight you torqued it.
Washers didn't work for me.
