Myles, sorry for the delay. Yes, the D4A is the direct replacement for the D3L. Actually slightly more BTU. You will need to order it with an endhood that fits your ducting, probably 3" on your old D3L. You do not need the truck kit, only the main harness. All your old stuff will work(fuel lines, exhaust, combustionair intake muffler) You do need to make sure you get the fuel pump plug with it as the new fuel pump, which you have to use, has a modular plug on it. You need the plug, and the terminals that come with it. You need special crimpers to crimp the terminals onto the wire, but you could crimp the terminals on roughly with pliers and then solder the connection. All the wires on that harness are plain copper, not tinned as in the marine kit, so you would want to solder all connections, and then spray them with a waxy prtectant eg. FLUID FILM or dielectric grease. The truck kit has an exhaust, that is too small for your thru hull and exhaust elbow. If your existing exhaust is ok, you do not need a new one, and if you do you need 30mm, not the size that comes with the truck kit. The truck kit also comes with a muffler, but it is not suitable for a boat as it is not sealed. It is meant to go under a truck frame. There is a marine muffler available for the D4 that is 30 mm and 5 ft long, and flexible double wall construction. It can be cut down to as little as 26" if needed. It does a fantastic job of muffling, but has a list price of $274.00. MSR in Wash. has a rigid welded muffler about 24 " line that has to be spliced into the exhaust.It retails for about $130.00 It need to have a support bracket mad efor it as it weighs a few pound and need to be supported. You would need 2 more 30mm clamps for that. The new exhaust clamps are made of S/S not like the old ones that rust out. Seal all your joints with muffler cement, or high heat silicone to be safer. Make sure you put in a breaker in the power harness near your batteries, it should be 20 amps. Blue Sea makes one it is a push button resettable one with a threaded shaft for panel mounting. The truck harness is light gauge wire, so cut the power harness part as short as possible for a lower voltage loss. The best controller to get is the Digitroller, it has a digital display and will also display fault codes and you can unlock the heater if it lock out on too many overheats or failed starts.