At 60mph my engine revs at 2800RPM and at 70 it's 3200RPM. My qustion is which condition would give the most miles from my engine.
60 MPH assuming you are in the same gear at both speeds.
At a constant gear, your engine turns at a fixed ratio to the distance you travel. So if you travel from A to B your engine will always turn X revolutions, regardless of speed. In other words, the piston experiences the exact same number of cycles regardless of 60 or 70 mph
Air friction increases exponentially, so at 70 mph you are using more fuel per engine revolution, and thus increasing the load on your engine. This load is transferred through the pistons, bearings, crankshaft, etc all the way through your drive train, including your camshaft. The tension on the timing chain increases, the load on the lobes of the camshaft increases and these things wear down your engine.
Since you are comparing two speeds where the engine is in the same temperature range you do not have to be concerned about under/over cooling. This comparison is not the same as going 1MPH vs 70MPH
While @Robert-Ryan is technically correct, I'd be surprised if there was a measurable difference in engine lifetime. Engine wear is dominated by cold starts. Once the engine is thoroughly warm, engine wear is minimal. If you want to maximize engine life, minimize the number of short trips where the engine doesn't get warm, or invest in an engine block heater.
Don't know if anybody has documented this, but I think it is self-evident that slower speeds and lower rpm's will result in greater longevity. Imagine driving your car at a steady 5000 rpm, and consider it's lifespan compared to 2500 rpm.
It depends on more factors. While generally lower RPM will save fuel there are cases, like when load increases due to a hill, where higher RPM can result in the engine working easier and lowering fuel consumption.
Likewise, being at the lowest possible RPM or highest gear is not the absolute way to get best MPG always.
Keep it under 60mph to reduce wind drag and you will surely see large benefits.