If you want more theory, be happy to type it up sometime. But as for your questions specifically...
The more radiating length that is above your roof line the better. The body of your truck is doing a couple of major things as far as antennas go and none of it is helped if the antenna whip is physically close to the sheet metal for relatively long lengths.
OTOH, I've mentioned this before and will repeat it, you have to come up with an antenna system that is workable and livable as much as anything because even an ideal antenna that is a pain won't be fun. Just understand that there are definite reasons that everyone says to drill a hole in the middle of the roof, that is absolutely the best location to install an antenna in all cases, but really very impractical for anything other than VHF and UHF unless you are really dedicated to mobile operating. So you do the best you can within the limits you have.
As far as using aluminum, that is fine. Make sure it's grounded just like any other conductive material since it's physically close to the antenna and will try to become part of the antenna and you don't want to be making too many stray caps and inductors.
Regarding grounding, use straps if there is any question. Unless the path is absolutely bomber it is likely that it will have some 'impedance'. Notice that I did not say 'resistance', because a fair to even good DC ground will probably not be a zero ohm impedance path, which is the combination of resistance and reactance (made up of capacitance and inductance). So a DC ohmmeter isn't necessarily going to tell you the whole story.
Other than the frame itself and large sheet metal pieces there are very few excellent RF conduction paths on your vehicle (read up about the 'skin effect' for EM fields), so ultimately what you have to do connect smaller things with braid to larger things and then make sure all the larger things are well connected. This is termed 'RF bonding' your vehicle. What you need to do is make sure a significant part has a good braid connection to its nearest large piece and then that larger piece has a couple of braids to the frame. Put a braid that bridges the swing arm to the bumper (the pivot might be OK for DC current but is terrible for AC), then a braid or two from the bumper to the frame. The only way to know that something is an excellent RF conductor is if it either has multiple continuous lengths or welds longer than say 6 or 12 inches, so if your bumper is painted and bolted it probably is a horrible RF conductor despite maybe being a great DC conductor.
'91 Toyota Pickup
'09 Kawasaki KLR650
'12 Gunnar Rockhound 29
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