And even though it does still get hot, that's pretty much the coolest part of the exhaust by the time it gets back there. So it's not like it's boiling the gas inside.
So I say it's ok too, but it also just makes good common sense to make a shield/barrier of some kind or another. Anything simple that reduces what heat does get back there is nothing but a good thing.
It's those long freeway drives that usually end up heating things up I would think. You've got a cooling wind under there, but you also have the time when the exhaust is seeing the most heat production up front. So better safe than sorry?
While the tank is full-ish I can't see it being an issue, but if you had an almost empty tank and on a vehicle with an already well known vapor lock issue, I'd go to the trouble and make a shield too.
However, truth in advertising I ran my '71 about like that on the passenger side for years (true dual exhaust, but both tubes on the passenger side) without any issues either. Even at extended highway speeds.
Looks like you have duals with the tubes on both sides all the way back?
Paul