Measure the width of the hacksaw blade, then go to the hardware store and get a drill bit that's for drilling through metal, in a size that's slightly larger than the width of the hacksaw blade. The aluminum post is a fairly soft material, you shouldn't have any problem drilling into it (especially since it's already got a huge pilot hole), even with a 12v drill. Once that's done, insert the blade into the hole, and begin the process of cutting the post. Again, aluminum is soft, it can be done. I've seen my grandfather cut galvanized steel using a similar method in tight places (he was a plumber). "It's a slow road," he'd say when he was about half done. Then he'd hand the blade to me.........What you're seeing with the thickness of that SR Laprade post is the difference in the aluminum alloys they used back then vs. the more refined stuff they use today. I've got a newish Thomson post on one of my mtb's, the post is taper walled 7075 alumuminum, and the tapering is to insure that not one ounce of material is used that's not needed. A definite contrast to an SR Laprade, for sure. And I think the flutes are for added stiffness, not weight savings. That, and maybe the cool look they give, I don't know-