Even the F/L series had relatively low wing bomb loads, it was only from the M onwards that the P-40 began to get used as a more serious fighter bomber and the late N series is a chapter of its own in that regard. The sometimes seen 3x 500lb maximum bomb load only applies to the last few P-40N production blocks.
Field modifications, on the other hand, are a different story but I wouldn't consider them a norm as the aircraft very likely were pushed to, and even beyond, the safe limits of what the wing could take before breaking.