Not sure how exhaust flow would effect RPMs at 70MPH in a manual vehicle or even in an automatic if torque converter was locked.
Its straight mechanics, even if engine was off coasting down a hill in 5th at 70mph engine RPMs would be whatever rear ratio and tire size calculated
If there was an engine/power issue then you would have to press gas pedal down more to maintain that 2,700rpms
I would check to make sure you have a 3.73 rear ratio
BLOCK FRONT TIRES
shift trans to Neutral
jack up rear axle
Mark drive shaft and rear tire
Rotate drive shaft 3 full turns
now slowly rotate it 3/4 of a turn and mark on rear wheel should be back to where it started, 1 full turn of tire
If you have to turn drive shaft past 3/4 to 4 turns to get tire back to 1 full turn then you have a 4.10 in rear differential.
Rangers came with Tuned exhaust, which means exhaust manifolds or headers are "tuned", which ever you call them.
Ford used mid-RPM tune
After market headers are usually made for low-RPM tune, although "racing" headers are made for high-RPM tune.
The "tune" means the diameter and length of the pipes from the head ports to the Collector(larger pipe they dump into) are designed to create a lower pressure at the head port at a specific RPM.
The velocity and the change in pipe diameter causes the lower pressure, scavenged power
And what this does is to reduce the amount of power taken from the crank to push out exhaust for a cylinder, so more power for rear wheels.
The Lower pressure "pulls" exhaust out.
So the closer you are to the "tuned" RPM the lower the pressure at the exhaust ports and the more power you will have for rear wheels.
When you change the size or length of header pipes then you change when or even IF you get/create this lower pressure.
Popular MYTH comes from this
MYTH is "My engine must need back pressure", no 4-stroke engine needs back pressure, lol, 2-stoke yes.
Where it comes from is that people would put larger header pipes on an engine and LOSE power, so they assumed the smaller pipes were providing back pressure, they weren't, they were providing velocity to create LOWER pressure at the head ports so more power was left in the engine.
After the collector you can do pretty much what you want as long as you don't go smaller, larger pipe won't effect engine power either way, but you can get a better sound.
On dual exhausts you can add "H" or "X" pipes to further scavenge power, these lower pressure in exhaust system because of opposing exhaust pulses from opposite banks of the engine, ain't much but ain't 0 either
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