This is a bit of "apples vs. oranges" but may help a bit: A few years ago I needed to reinsulate and recarpet the front cab area of my '85 BlueBird RV (this is basically a school bus chassis set up in this area...so like your truck, you have room to work, weight is not really an issue, drivers area not really designed with "sound control" as a priority, and the engine is right at your feet putting out heat and noise-so you need good stuff).
In the interior, I found marine engine room type insulation (dense closed cell foam with a asphalt layer and foil facing) from JC Witney and West Marine were excellent to work with, apply, and gave good results in both sound and heat insulation. This comes in various thicknesses...I used 1/2" almost everywhere except the interior of the engine compartment/firewall where I used 1" in most areas and 1/2" where access was just not there. Once I got this stuff in, you could really tell the diffrence in noise level and heat transfer. Secured with 3-M heavy duty (99?) spray glue and some mechanical screws/washers. Seams sealed with mylar tape. This type of insulation looks good raw, but could be covered with a fabric layer to finish off if exposed in a driver/crew compartment.
But, prior to this, I had used spray foam to seal up seams and cover the underside of floor panels as insulation....what I found was good heat insulation....but minimal sound deadening. I think due to the light weight nature of the foam when cured...sound deadening needs a range of density to control a range of sounds. Also, upside down, this foam approach is a ***** to work with...and getting this stuff off you is tough. I would use foam to seal up, but rely on a good multi coat application of standard undercoating for exterior exposed areas needing weather protection and sound deadening.
One caution would be to avoid having your insulation "trap" mosisture against your steel, inside or outside...causing unseen rust issues. For your application you might even consider having your floor insulation removable should you need to dry out or inspect area.
Good luck!