Enlarging the head bolt holes will allow you to bolt the heads to the 403, but you may want to install larger valves as well. The #7s came with 1.880" intakes whereas the 403s had 2.000" intakes (which are the low-perf big block valves). While you're at it, you may want to install the larger 1.625" big block exhaust valves as well. The problem is that by cutting the #7s for the larger valves, you'll loose the factory induction-hardened valve seats.
Also, I assume you know that the 403 had big block sized 83cc chambers originally vs. the 68cc chambers on the #7s (which I assume is the reason you're doing the swap).