Lots and lots of stuff For one thing the LT1 engine uses a reverse cooling setup.That requires a different type of radiator. Than there is the fuel system and lets not forget the computer.And more plus that . But that alone are enough problems to try and solve. It would be easier to just stick with an older 350 running an HEI ignition system and a good old fashion carburetor.
A good 350 with HEI and a Quadrajet would be easier.
You don't need a different radiator. The 1990's LT1 reverse flow cooling means the water pumps through the heads first instead of through the block.
It will bolt to a smallblock transmission unlike an LS engine.
If you want an automatic transmission you'll need a 4L60E of the same year as your donor engine and PCM. Manual is a matter of programming the PCM with a manual transmission program... F-body or Corvette.
The wiring is usually what puts most folks off. What you will have to collect is the whole donor ECM to engine/transmission harness. You used to be able to buy an LT1 stand alone harness but I usually get a wrecked/rotted donor car.
A MAF Tuned Port Injection setup out of a 3rd gen F-body or C4 Vette mated to mid 1990's GM Vortec truck cylinder heads and a Scoggin Dicky TPI-Vortec lower intake would be easier to graft onto a 350 long block in your truck. While the TPI tends to flatten out at higher RPMs it'll have a lot more low end grunt than the LT1 and that's what you want in a truck.
The later speed density systems will handle higher RPMs but they were a lot less forgiving of variations from the original engine specifications than the earlier MAF TPI setup.
I usually ran 2-1/4" or 2-1/2" H-Pipe exhaust and headers on my TPI swaps. More than 2-1/2" and you start giving away low end torque for very little high end HP.
Any way you slice it if you run fuel injection in a squarebody you'll need the RV series tank(s), Sender(s), and the appropriate fuel pump(s) for your injection system. The tanks and senders are stock 87-91 RV series squarebody truck parts. Fuel pumps are nearly stock. TPI and TBI run lower fuel pressures than the LT1 injection systems so you don't have to give up the 65PSI Pollack valve and second fuel tank on the dual tank trucks.
The L05 TBI systems from the 87-91 RV series are an OK choice. Pretty decent fuel economy and dead reliable but not super exciting HP and Torque numbers.