Spacing? As far from each other as is physically possible with the least amount of compromise on ground plane. Clear as mud, huh?
Low band and UHF can be placed on the same side, and the puck antenna and the VHF can be placed on the other. That combo will make the spacing less critical, but if you use 1/4 wave antennas for the UHF (~6.5 inches) and for the VHF (~18.25 inches), their placement in respect to height becomes less critical.
If you're placing these antennas on a large ground plane at sufficient spacing, and you're using a repeater or simplex system that isn't 30 miles away, you'll probably not suffer a bit using 1/4 wave antennas vs gain antennas. Gain antennas properly placed on a good ground plane will be superior, but they will also be more narrow in bandwidth and will be more sensitive to placement near other metallic objects.
No matter what type of antennas you choose, make sure the antennas are spaced as far as possible or at least something more than a 1/4 wave length away from each other and other metallic objects.
The low band. Well, the low band is going to be a compromise no matter whether you use a base loaded antenna or a 1/4 wave because there's no way you can get the antenna a 1/4 wave length away from anything on the truck. The low band will really be sensitive to the proximity of almost any objects closer than a quarter wave length and will not like small ground plane mounting.
I suspect you'll be locked into using a base loaded low band antenna in order to get it on the roof. The most forgiving low band base loaded antennas I've worked with are the Larsen NMO series, and I don't find that the versions with a spring will survive any better than the straight versions. Neither likes tree limbs or low door heights. Whatever will bend the straight one at a sharp angle will also destroy the spring in only a few hits. Note: it's the little wire inside the spring that suffers from repeated stresses, especially the quick and close ones, and once the little wire is broken, the antenna will cease to work well or tune. The spring really just means it's not likely to be damaged on the first strike.
Here comes the most important part of frustration relief: if the roof of the truck is aluminum, and I'm pretty sure it will be, it'll be as much as 3/16 inch thick. That'll mean that if you use NMO mounts, you'll need the ones designed for thick roof mounting. Otherwise, if you just get the standard mounts you'll have 4 mounts that won't work....period. There are two thick roof mounts: one for up to ~1/4 inch or so, and one for maybe an inch thickness. The ones for an inch or so are terminated with an SO-239 connector and will require a PL-259 on the antenna end of the feedline, so you'll need room on the inside of the roof for that connector and the extended connector portion of the mount plus room for the bend in the cable. A good right angle connector does the heart and the antenna mount good here and keeps involuntary vocalizations about somebody's mother to a minimum.
Hope some of this helps.
I'm looking at my model CF-416 DUPLEXER. It has three coaxial terminals, one marked "COM", one marked "1.3-170 MHz", and one marked "350-540 MHz." This is the model I have used for vehicular repeater installations and in my personal cache of equipment.
