Question about the difference between R-6 and V-6 engine?

My friend and I have a debate going on now. I am a big fan of I-6 engine and built for trucks Mustangs and classic. I'm working on an I-6 300 engine to be put on a classic Mustang! I told him as much engine, but one drawback is that not rev as high. He says I'm wrong and the I-6 engine rev higher than V-6 engines. I think he's wrong, because a I-6 has a much larger crankshaft, and therefore more weight rotation, which means lower revs. He thinks that because the pistons are in a row, which is higher rev. Who is right? The engines higher speed, I-6 or V-6?

an I-6 rev slower than a V6, not by weight, but due to torsional forces on the crankshaft. Actually not damage the crank to rev higher, but it can damage the bearings. Some large engines tried to compensate for this problem, putting in two thrust bearings, instead of the standard. This is also why the I-6, in general, produces a torque more efficiently than a V6, because the forces of torsion load rotational force. The shorter the V6 will be able to spin faster, and more since the torsional loading smaller but will not turn this into couple of the most efficient manner.