hello and welcome.
for the floor and front cab mount replacements I really recommend first ensuring the door hinges are in good shape and the door is also not cracked in the hinge mount areas etc. then fir the doors to the cab, don't worry about how the doors fit against the fenders as that is another step past the doors fitting the cab. this is important because the front cab supports are tied into the forward door pillars and the step. if you replace the cab supports and don't have the doors supported and fitting well then you are really asking for trouble when it comes time for door fitment. not sure if you have any history with the task force trucks but the common issue with them is that the grille in front of the windshield is common to the cowl and side venting systems on the cab. the cowl lets air into the vents by your feet but it also lets water in there as well. the water will normally drain out the bottom of the side cowls but the hole it has there is pretty small so any leaves, pine needles, mouse house supplies etc will plug the vent. then when water gets in the cowl there is nowhere for the water to go except evaporate, if it is a small amount, or, for the deluge rains or snow build up, the excess run off build up inside the side cowls until it reaches the height of the vent doors and then it runs out onto the cab floor. eventually this leads to the floor rusting along with the step well and the forward door hinge pillar. since the door hinge pillar is also connected to the step, the fender rear lower mount AND the cab mounts you can see how this all works together for a disaster if you simply cut out the old rusty stuff and weld in some new parts. on my old 57 gmc I found the front hinge pillars were toast along with that whole corner of the cab, floor, steps, rockers, fender mount, pillars, firewall etc. it was bad enough so the floor actually had a low spot under the pedals because the cab mounts were soft and non supportive anymore and so the weight of the front of the cab/firewall, doors etc was enough to make the floor sag. if you check all that stuf out and there is no other issue other than the cab supports and and floor work then that is great. I would go ahead and fit the doors, then with the doors on and closed, weld in some cross bracing in the door openings and then across the cab from side to side at the front of the door openings. do it in such a way that you can still do the work without too much trouble. then when it is all done fit the doors back on and remove the bracing. this should ensure the cab can't move out of square when you remove the doors that are sort of holding things together. below is a link to the factory assembly manual pdf that someone was good enough to post. I would download it and save a copy for future reference.
https://www.trifive.com/d1/55-59Assy.pdf
here is another link to a site with some good info
http://chevy.oldcarmanualproject.com...esto/index.htm
here are some pics of how I braced mine except for the door fore to aft bracing. I haven't found that pic yet. when I did that part I made a horizontal brace high enough off the floor so I could slip a couple of 2x4's under it, on edge for strength, and I was able to "hang" the cab from those by placing the 2x4's on sawhorses. this allowed me to work under the cab floor without anything in the way. I also cut my firewall out because it was pretty rusty where it met the floor and I wanted to fab a new one to fit around the LS engine and trans. there is also a pic of the front hinge pillar from the front side with it all taken apart and taken off the frame. it shows the crushing and consequential sagging problem. the pillar gets soft and crushes under the weight of the front section of the cab. there is a pic of the front of the cab with the main cowl part removed. it is hard to see but there is spot welds drilled out and there is a gap between the inner and outer section where air is supposed to flow down to the vents. below the section with the surface rust. you can also see the inner cowls are rusted out where the front fenders mount at the top rear of the fender. that is also really common where I come from anyway.