The biggist weak point i know of is the cooling system, thats where i hear about problems. IMO never bore a 403 more then 40 over, or you will drasticly increase ur chance of over heating.
Mine is running flat top pistons, giving around 10-1 compession, which lets me run any type of gas with no detination. And I have a slightly more agressive "truck" cam then you which is giving me much more tourqe and a bit more hp. (which is perfect for my tall rear gear)
Deffinitly get a dual exhaust with headers, the 403 needs a lot of room to breath, my motor was first in a buick with with the stock exhaust, which it ran good but when it went into my cutlass with headers and duals, it completely change the way it runs, and woke it up.
My 403 now has around 20,000 miles on the rebuild and it runs as good as ever
As for the heads, i kept the stock heads, but the heads and the block where each milled 0.010 (0.020 combined)(BUT doing that much required some creative milling for the intake to seat properly
From what we looked at, the 350 heads had to small of chambers for the 403, the pistons are wider then the combustion chambers. Which may increase compression, but it limits the spread of the burn on the pistons.
1987 Cutlass Supreme, with an olds. 403, punched out .040", Deck & heads milled .010" each, Crane fireball cam, edelbrock intake & 750cfm carb, th350 trans, w/ higher stall torgue converter (approx. 1000rpm over stock stall speed). And a 2.14 rear gear. (yes I said 2.14)