With that combo (32 tires and 5 speed trans) you will probably only be pulling about 2000 rpm at 70 mph. At those speeds your engine will probably be lugging and might even start pinging. Is that bad? Here's what Tom and Ray Magliozzi had to say.
TOM: When you lug the engine when you try to accelerate in too high a gear you make the engine work harder than it's supposed to. Just like you would work harder than you're supposed to if you rode your bicycle up a steep hill in 21st gear as opposed to first gear.
RAY: When the engine is over-worked, it overheats. The way it overheats is that the temperature inside the cylinders gets too high. When that happens, the gasoline-and-air mixture which is supposed to detonate when the spark plug fires detonates unpredictably at other times, too. That's called pinging when those extra detonations make a sound like marbles rolling around in a coffee can.
TOM: And under the worst of circumstances, you can also get piston slap, which is a "clunky" noise. Because you are getting uneven combustion inside the cylinder (the detonation doesn't necessarily happen at the right time or in the right spot), the explosion can force the piston off to one side when it descends during the power stroke. When that happens repeatedly, the cylinder walls get all scraped up, and over time, your car turns into the proverbial oil-burning heap.
I'd say either go with smaller tires or change the gears to keep the rpm's in the mid 2000 range. You can drop from 2700 at 70 to 2400 or so without to much problems. Any lower and you could be courting problems.