Keys to slimming down your xbox -
1.1 - 1.5 make best slims, 1.1 has no daughter USB board, but has the biggest TSOP for flashing. BTW, TSOP flashing is a must unless you absolutely have no choice but a chip.
1.1 has the older connector head, which isn't as convenient as a 1.2-1.5
after modding via TSOP (I can give instructions and links if you need them) rip all of your games to your hard drive. At this point I strongly recommend either getting a SATA -> IDE converter or a really big laptop IDE hard drive. Laptop IDE's are harder to come by and are smaller, I bought one the other day for $50 and it was 160 gigs. For $100 I also bought a 1 terabyte SATA 2.5 in HDD.
So yeah, rip all of the games to your hard drive. Once you have completed this process, I also recommend taking the time to back them up to your PC so that they are available should your hard drive fail. Remove the DVD drive from your xbox and ensure that your TSOP's reference file (I am presuming you would use one of either the x2 5035 bios or iND bios) indicates that DVD check is disabled.
With a little work you can put together a shortened IDE cable, or properly folded, and build a custom holder for the hard drive. I recommend putting it where the DVD drive used to go. You can use the ICE BERQ PRO heatsink/fans on the CPU/GPU. PicoPSU @ 250w will be plenty to support power wise. That cuts your height down considerably, and gives you an extra two inches of space where the old PSU used to be.
So now you will have an xbox that boots without a DVD drive, capable of being considerably shrunk down in size, and is better cooled. All without a modchip
you can also build a BIOS switch if it is a 1.0-1.1 and load both BIOSes on it if you want, but I don't really see the point.