Those mirrors are just one of the 'day two' (or three or four) modifications represented in the moebius F100 kits. The front sway was not offered as an option until the '73 F250/350 and not on F150s til '75. The 4bbl carb truck 390 wasn't offered til '75 and the large power brake booster didn't come along til about '77. A more accurate booster could be found in the revell mustangs or '70 torino.
Doubt there was any special reason other than that was the way it fit best when the basic body shell was engineered in the late fifties. The '66 restyle gave the falcon/fairlane/comet bodies a bigger engine bay with more room to mount these extraneous items.
Another consideration for us north of the medicine line is that sometimes extravagant expenditure known as 'shipping' There still exist some local suppliers that sell at a fair price that beats the toll charged by most online suppliers and when I find one, I try to support them. The chance to talk with fellow modellers is a plus at times too. Even with extra bucks spent, the bridge-building and friendships developed can be worth it. This even applies to trading stuff across the line sometimes.