This is a subject that could end up in the tech archive.....if it gets noticed by anyone here. The front vent window posts on most HARDTOPS are made of a Zinc based DieCast metal,not a real Aluminum alloy. When you chop the top, you will have to shorten them as well... This has been a problem so I want to share a few bits of information here on the subject. The ones I am showing here are from a 50-s GM. It was given a 3" chop. First you need to plot the cuts where the MINIMUM amount of "welding"/-brazing ,is needed. I say this to save you trouble as this is not an easy task. I had done chops on these as long ago as 20 years ago, but not really much lately- so I needed to refresh my own memory as well... I cut the top off the posts and removed the 3" as I did on the adjoining body posts. The problems are multiple here -as it's a combination of things that makes these a bear to shorten... The vent post's metal ,is very touchy -in that -just as it reaches the melting point it is VERY NEAR the point of just melting and dropping off as a big splatter of molten metal,leaving your post an inch or two- too short! I remembered using a propane torch the last time I"welded" these. I had misplaced my old one and had to buy a new one a few days ago... sadly it is not a very efficient one as it would not do the job at all. I next used my acetylene torch[I use Propane with my OXYgen]. This proved to be hot enough at almost it's very lowest possible flame pattern . I used a special alloy brazing rod that combines Magnesium with the rod ...it was about 5 dollars for two 10" rods. Rods are Made by BERNZOMATIC.....available at most hardware and farm stores... they do not go very far in the process -so buy several. I drilled the two peices to be joined -and that was very tough as the only "flat" area is very small at best. This was done so I could "pin" the two in position.....using a screw in these 1/8' holes...I used one that I could bend slightly so as not to break it while final alignment is achieved..... It also allows you to install it on the door for fit.... This is also necessary because the two sections are not straight or square in any of their surfaces. That basically prevents you simply clamping the two with clamps or vise grips well..... I then heated very slowly the entire peices together for a few minutes, before concentrating the flame on the splice.... take your time and hold the heat -back -when the posts start to become molten... as stated above it is very touchy and can easily ruin your posts completely..... I then flowed the rod in the splice[1/8" wide] which ends up bein a very nasty looking glob of metal! I used cut off wheels to slice off the majority of the lumps -then a 24 grit sander to sand the area smooth.. It clogs a lot so several discs are needed. These are welded although a long way to being ready for a plater..... Hope someone can use this info here -it seems not a lot of tech is posted any more.... I wonder why that is?