You can pull the body off the Denali and swap the cab onto it.
A few observations:
The Denali has a 116" wheelbase (or 130" if it is a Denali XL). That is a fair bit different than the 125.25 of the 3600. If you end up with a flatbed, that difference can disappear into the noise. If you want to put a traditional bed on it, you will have to either move the fenders or adjust the frame length.
The Denali is 78" wide compared to the ~73" width of the 3600. That will manifest itself in the stock wheels sticking out of the fenders. You can get offset rims that will tuck them in some, but 2.5" per side is a lot.
The fuel fill port on the Denali is on the back, driver side. You will need to come up with a support for it or routing to the stock fill port on the cab.
The 3600 master cylinder is under the floor, where the Denali setup expects it to be on the firewall. The easist solution is probably to get a firewall pedal mount and convert the cab using that.
The stock 3600 steering column isn't compatible with the Denali box and the denali column won't fit/mount in the cab. Some kind of custom mount/shenanigans will be needed to hook up the steering. I have read good things about using 80s S10 columns for that conversion on 3100s. That might also work for you. Alternatively Speedway sells a well priced universal column with your choice of ignition location and shifter location.
Electrically, you can separate the cab and the chassis at the bail connectors on the fuse block, then hook your 3600 wiring into it there. That is pretty straightforward to do.
As far as gauges go, the oil pressure sensor on the denali goes straight to the gauge, so you could replace the sender with your original one. There is a spare coolant temp plug on the passenger, rear head that you can put your origonal temp sender into. The easiest thing to do for the speedo is get a digital->mechanical converter to run your stock speedo.
Is the Denali 2WD or AWD? The AWD ones have a controller that will need to be transplanted with the engine harness. I don't know offhand if it uses any inputs from the BCM or not.
When putting TF cab on a GMT800 frame, the inner fenders need trimmed for the shock mounts and the bottom/back of the cab needs trimmed to clear the wider frame. The AD trucks are a bit narrower so you will probably have to do something similar.
The ABS and Traction Contol biasing is different between the truck and suv platforms, so you might consider getting your ECBM reflashed to the truck variant.
If you want the cruise control to work, you need a later model dual pole/dual throw brake switch.
The heater core in the 3600 is going to be a lower pressure rating than the LS cap. The easiest answer there is to replace the heater core with a modern one that can handle the higher pressures, but I don't know offhand what is available off the shelf.
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Bottom line: Having the swapped frame gets you a lot of benefits in modern technology, parts availability, etc. It is a non-trivial effort though.
As long as you go into it with your eyes open, have fun and we are here to help you out along the way.
