You can drop in pushrods without removing the heads by using the OTC tool that goes into the injector hole and pushes down on the valve bridge.
According to Geoff Bardal, who owns Colt Cams, there's no reason to drop in 6.4L pushrods into a 6.0L The shorter pushrods were needed because the initial batch of 6.4L motors used the 6.0L original length pushrods and were having valvetrain issues, not the 6.0L. The camshaft centerline is higher in the 6.4L motor, closer to the heads. International stopped selling the original length 6.0L pushrods to prevent any later cross motor issues.
I've measured the lifter travel; there doesn't seem to be any bottoming issues. If the 6.0L had an issue with pushrod length, the motor would have a history of burned exhaust valves and bent valvetrain. Yet we have trucks over 200k miles, none of this.
Any motor can have bent pushrods if the motor sits and the valve stems and guides rust up, which does happen with the 6.0L. Lifter's "pump up" because the slow retracting valves provide clearance for the plungers to take up the false slack.
Eating a lifter has more to do with poor oil and coolant maintenance; often a motor that has had a clogged oil cooler in it's past. The lifter bearings get damaged or the cam lobes from the higher surface pressures from inadequate lubrication IMO. When disassembling well-used lifters, I've found fretting of the needle bearing surfaces, and in older motors pitting on the cam lobe surface where the hardened surface is popping off.
But even trucks with good maintenance can have a lifter/lobe failure, typically in the 250k mile range. It's a highly stressed valvetrain.