RE: Global Defense Force, -48 is totally fine, nothing is cut off at top or bottom at all!
As for Runabout, I find -48 is the best for it too. This cuts off some of the bottom, but all you lose is total time + score. You can still see your speed and most importantly the top of the screen you can see your time remaining and the map fully.
SO, as for BCV:
This is complex to explain so I'm not actually going to do a full tutorial, but I *DID* manage to get it to work properly! It requires a LOT of faffing about though and a spare memory card.
Basically, what you have to do is find a way to install "FreeMcBoot" on the memory card (as I have a chipped PS2, I copied the files of Free McBoot to a USB, then burned a uLaunchELF ISO which would allow me to access the install files on the USB from the PS2, ran the disc, found the usb, ran the FreeMcBoot ELF). There is a "noobie pack" which has all the FreeMcBoot stuff you need if you google.
With FreeMcBoot your PS2 will launch into a new browser direct from the memory card(!) I have no idea how it does it, but it works. You now (or you can do this at the same time) need to install a thing called "Graphics Synthesiser Mode Selector" (usually just called GS Mode I think) on the memory card too. This is a thing you run which allows you to select any video mode for games!
To run BCV now*, what I do is make sure the FreeMcBoot memory card is in, then put my original BCV disc in the machine (tray open). the machine boots into FreeMcBoot, I launch uLaunchELF from that, then launch GS Mode from that, select NTSC and my y+x fix requirements, close the disc and exit GS Mode into the browser.
The game loads and is fixed to my needs!
*I actually have it set up with some configuration files (also found on the web) which means I just put BCV in the drive, and when I turn the PS2 on I hold L1 and it boots right into GS Mode selector. I *still* need to select NTSC and y+x fixes every time, but it's a lot faster.
Using this method you can run ANY PAL PS2 games on an NTSC machine from the original discs with a bit of fiddling each time you run it, but it's a one-stop solution.
Final note:
I think GS Mode selector actually naurally y-fixes for PAL on NTSC, so I don't have to set it. However for whatever reason I am required to x-fix BCV and Snowboard Racers 2 to -48 to center it on screen. Weird I know.
Oh and if you don't already have a chipped PS2, you need to use some other method to get FreeMcBoot on your memory card, but it's possible with a variety of methods, really whatever you use to boot your PAL games as it is.