With 4.56's you'd be about 3000rpm @ 70mph, 4:10s would put you about 2800 @70. With Th350, you can probably get by with 4.10's just bog down a bit as expected with biggo tires. I'd also consider carrier ratio size breaks in your decision depending on your axles. Example being, If I was running a 14 FF, and I was on the fence of 4.56 or 4.10's, the carrier break is 4.56 on down to lower gears, or 4.10's up to higher gears. If I thought there was a chance I might want to go a deeper gear in the future with even taller tires, I'd just go the 4.56's now so I wouldn't have to buy carriers again in the future. If I thought, I might be a bit afraid of the high rpms, I'd just go with 4.10s now and I did decide the 4.10's were to much, then I could always step taller to 3.73's without replacing carriers.
Another thing to consider is future axle upgrades. I was currently running 10bolts, I'd go with 4.10's instead of 4.56's. Reason being, I know 14b Full Floats from the factory are fairly common. I also know it's hard to kill a 14b FF. So if I go 4.10's now, I find a 14b FF with 4.10's in the future that are pretty common, then all I have to do is swap my front 10b to 8 lug hubs. And the chances I was to find a 4.56 14b Full Float is because I purchased a set of axles from a CUCV having a Dana 60 front and 14B FF w Detroit Locker in the rear and I'd just have to move spring perches and shock brackets and bolt it in.