cleaning up exterior

[CNC]

First off hey all love the site. Been lurking for awhile but now I am finally able to post.

I have an 81 long bed with the 91 Sub front end, that i plan on doing some simple mods on her like converting to power doors, custom headliner, and maybe a smooth dash. A couple that I am thinking about is shaving the cab seam and drip rails, The cab seam I know i have to take the rubber stuff out of it that is no biggie and then weld it up, are there any negative side affects of doing this and also the drip rails if I shave them off how does it effect the rain run off? Are there any tricks to keep it from running into the cab?