I'm not a Ford guy, but I have done a few frame swaps, and I have done a few suspension swaps on to different frames. Both can be pretty challenging experiences each, by themselves, and now you are thinking about adding the two together. Not saying you can't do it, but then I don't know how much experience you have doing things like this. I'm suspecting not much, so I will suggest where I start every project like this.
Start by parking the 86 truck side by side with the 59, with both front wheel dust caps sided by side. Then open both hoods. You want to look closely at a few things before you jump in.
1) How does the motor sit in relation to the the front tires on both trucks? Some sit with the motor centered over the front suspension/tires, some sit with the motor behind the front suspension/tires, and some sit with the motor ahead of the front suspension/tires. That can make a big difference on the next points.
2) How the 86 motor wold fit against the 59 firewall is a biggie. That 86 motor may fit well with the 59 firewall, or it may sit too far forward, or too far backwards. Cutting the firewall for motor clearance is not unusual, but the deeper the motor sits into the passenger compartment, the smaller the cab gets, and the more complex everything gets.
3) How does the 86 radiator location match up with grill/front end of the 59? A lot of older trucks have pretty short front ends, and the cabs sit forward on the frame more. Could be a problem with both the firewall and the radiator clearance. Expect that you are likely going to have to cut the front end of the 86 frame off, and you will also most likely need to fabricate the radiator, front fender support, and front bumper mounting brackets.
4) Generally, swapping an older cab onto a newer frame tends to lower the old truck cab. The older frame generally sat higher then the newer stuff, so the old cabs were already channeled on the old frames. When those old cabs move to newer frames, the tend to sit inches lower to the ground. Adding the CV subframe may lower the front another few inches.
5) Walk around to the rear axle. Are both in the correct position so the tire will be centered in the rear fenders? You may have to modify the rear of the 86 frame/rear suspension location so the rear tires will be centered in the 59's wheel openings. You may also have to cut the back end off the 86 frame and fabricate the rear bed side mounting and rear bumper brackets.
Parking the two vehicles side by side will give you an idea how big of a project you may, or my not have. Adding in the Crown Vic subframe may make some stuff easier, or make it all a lot harder. That subframe may alter the motor to the front frame/front tires in a positive or a negative way, but may also allow you to move the front tire location on the frame for a better fit.
Next, I would start with looking into installing the Crown Vic front suspension onto the 86 frame, see if that has been done and if anyone has recorded the process of doing it. I would guess that process would most likely drop the front end of the 86 truck frame down a few inches, the CV subframe will probably be bolted to the 86 frame. That process would be much easier if the frame was not under a body, and without a motor/trans in the way. The motor & trans may bolt onto the CV subframe with factory parts after the clip has been added to the frame (and after the frame has been installed in the truck), that is why I would start there.
Once that is complete, then the next part is to put the "new" modified frame under the 59. Every frame swap has its own set of issues. The biggest problem with frame swaps is the relationship between the front tire fitment in the front fenders (and the rear tires in the rear fenders) and then how the motor location on that frame fits in relation to the firewall of the old cab. Its not unusual to have to cut the firewall for motor clearance. That can make things get a bit tight inside of the truck. Sometime over the course of a 27 year span, Ford may have accidentally made a bunch of parts that fit together like it was an intended event, but then there are those other times when it seems the opposite goal was in mind.
