I've run both those antennas on my 800MHz system. On 800MHz they are slightly shorter.
You'll have a hard time finding anything that will beat mounting the permanent NMO mount on your cab roof. Since these antennas are so short, mounting them anywhere else is going to impact performance. The short 1/4 wave antenna is going to be 3 inches tall, so unlikely you'll have a clearance issue on a full size truck roof, unless of course you need to grease up your truck to fit it into a small garage.
I've got a full size Ford with both a 1/4 wave VHF antenna and a 1/4 wave 800MHz antenna on the roof. I've tried the higher gain antennas and don't see a big enough difference to warrant putting up with the higher profile look of the antenna.
You can mount it lower, but you'll be impacting overall performance. Not saying it won't work, it'll likely work just fine for what you are doing. However, ideal location would be on the cab roof. Nice thing about these higher frequencies is that they only require a small ground plane. You could remove the center stop light on the cab ( if you have one ) and see if there is access that way. If there is, you can mount the NMO mount you have there. You'll likely be impressed with the performance improvement over what you are doing now. Getting the antenna out of the inside of the vehicle will make a big difference. Also, once you mount an NMO mount, you'll be able to swap out antennas for whatever you'd like.