First things first i must point out ive never prepped a cab for decals but was motor trade all my life so done hundreds of cars and applied varies strips and decals, In my humble opinion i would never apply direct to wood as no matter what every imperfection will show through (yep i know some may argue that they were originally to wood) but for me if i was going to all the trouble of stripping and rebuilding for decals i would want as close to a mirror as i could get.
I would do it this ways.
Strip old decasl off using whichever metho works best,ie heating peeling sanding etc,i would not use a razorblade scrapper as you will only dig out the cab leaving more filling to do latter,i would at all cost avoid paint strippper (nitromorse and the like).Once the basic strip was done hit it with some 120 to ease whats left behind off.probly do this by hand or might use a Da
For hand sanding always use a decent block,i would us a 3m rubber block where posible.There will obviously be areas and profiles that you have to use the paper in your hand .
Now the real work starts.i would hit it now by hand with some 180 (note no filler used yet ! ) once i was happy that it was looking reasonable its time for closer inspection.Now fill any dinks and dents with quality wood filler and leave to fully set,being impaitant and sanding to soon will only lead to more work in the long run.
Once filler is dry de-nib just the filled areas with the 180 and re check (do you notice still no paint applied yet !)
Time to get out the fine sand paper 240 or there about,hand sand the whole thing again.And then visually inspect,if your eyes and hands can see or feel any imperfections back to the filler and sanding stage.Once your happy that its all good time to get the 400 wet and dry out (use a good quality paper not cheap as chips ebay stuff) .Small tip hear,use luke warm water and add a squeeze of washing up liquid to the water and change water regularly.Time to add some paint, i would use a devilbiss on a compressor but a hvlp would do the job.(you could also hand paint but hvlp are so cheap its hardly worth it).Apply a coat of primer (i would probly use grey but would depend on colour of decals to be applied).Once dry give it a quick 400 over by hand and apply a second coat of primmer(you could at this stage use one of the many filler primers,depending on how good your previouse prep was).Once dry give it a quick 400 over.Now time for a dust coat of black ( a dust coat is like a speckle coat you are not looking to fully colour the grey primer merely as the title says make it look like you have throw a pile of fine dust at it) .Time for the 600 wet and dry. Now hand sand again ,hopfully you will achieve a nice finish however if after sanding ther are still areas of black dusting this are low areas and will need addressing with very fine filler , i would use a polyester stopper filler. Wet flat this once dry.Stick another dust coat on and then re 400.Once you are happy that there are no high/low spots its time for a top coat.Choose a colour suitable for you decals bearing in mind the decals wont get the extreme edges and possible other areas of the cab.Apply your top coat of choice,leave to dry and hit again with the 600. Now its second coat time,leave the second coat to fully dry,Flat and polish stage time,(my favorite) Time to change to 1200 wet and dry,again warm water and a drop of washing up liquid.At this stage its more polishing rather than flatting.(if you were going to leave the cab just painted i would use 2000 wet and dry and then give it a clearcoat or a go over with g3 compound) Ok the cab should now be spot on, You may think this is over the top (choice is of course yours) but quite honestly its not a lot more work doing it like this than doing it poorly.Now apply your decals to this lovely surface by whatever method the manufacturers recommend for their glue type.Once the decals are on it should look top notch.For me i would then give the decals a coat of clear (asuming they were able to take it without reaction,) and then a buff with g3.
Like i say just how i would approach the task,You could quite happily just rub the cab down,apply filler where required,re rub the cab down and stick the decals on.But i know i would not be happy with the finish as every single imperfection and wood grain mark will show over time,Whilst doing all the above i would also give the inside of the cab similar love and finish of with the appropriate colour,probably a matt/satin finish.