You can bolt the TH400 straight up, but you need a special flexplate to do so. The 5.3L's flywheel sits about .4" recessed as compared to the old Gen I & II small blocks.
Gauges are fairly easy too, there is a little adapter bolted onto the engine just above the oil filter that can be drilled and tapped for a stock sending unit, and there is a plug on the rear of the passenger side head that can pulled, then using an adapter that S&P sells, bolt a stock sending unit straight in for the temp gauge.
Engine mounts are just a matter of bolting on the adapter plates, and dropping it in.
Hydroboost shouldn't be too tough either. You may even be able to bolt your old power steering pump straight up! If not, just look for one from a 6.0L, or a 6.6L, both came with hydroboost factory. The pump still uses the same fittings as your 84 does.
You will need to get yourself an F-body exhaust manifold for the passenger side, as the truck manifold sits too far from the block and hits the frame. The driver's side fits just fine. Exhaust shouldn't be much different than a regular exhaust system.
If you don't want an emissions install you can eliminate the rear O2 sensors, cats, EGR and EVAP canister in the prom when you get VATS disabled. Unfortunately, PCM flashing is like 350 bucks minimum, and all they have to do is uncheck a few checkboxes on their computer screen, and spend 10 seconds re-flashing your PCM.
Fuel system shouldn't be half bad, need an inline pump that can produce at least 60 PSI of fuel (walbro 255lph would be fine), and a high pressure filter. There are adapters available to convert the factory fuel lines from their press on fit to a typical -6AN fitting, then just run braided down to your frame, and run that all the way back to your tank. You can also buy in-tank pumps capable of running the proper pressures / volumes, and use factory TBI lines, less the filter all the way up to the fuel rail, then buy a set of Saginaw to -6AN fittings from aeroquip to hook up to the -6AN fittings that hook to the fuel rail lines.
It will be easiest to install the radiator from one of the newer trucks, since it has a couple extra nipples on it for steam lines and such, but it is possible to use an older one, and return the steam lines to the water pump and such. The stock 3 cores provide plenty of cooling capability.
You can try and run the stock engine driven fan, or get a set of windstar / taurus fans to run electric.
If you get a 99-01ish engine, they are still cable driven, and the cable can bolt straight up to your stock pedal by slightly enlarging the hole for it on the firewall.
I think that is most everything... Might be forgetting a thing or two...
I'm expecting to have about 2500 into my swap when all is said in done, including the cost of the engine (800)
