Ok. I did this swap several months ago and heres my take on it-
You can use your 3.0 oil pan if you run the factory IFS (bolts right up) but you'll need to modify the pickup tube to fit the 3.4 motor. I had to cut off one of the mounts for the tube and trim & dimple the 3.4 splash guard slightly to properly clear the tube. I originally was planning to use the 3.0 2wd T100 pan & pickup but my application went from 85 4cyl solid axle to 88 6 cyl IFS. You will also need to replace the dipstick tube with a 2wd T100 tube and pickup in order to fit your 3.0 pan. There's a little bit of work involved in that depending on the casting year of the 3.4. You will need to drill a hole in the block if it's 99 or newer. If you plan to do an SAS you'll need the 3.0 2wd T100 pan & pickup. I would consult Mike at ORS about the particulars.
Regarding the harness, you'll need to use the 3.4 harness and harvest the plugs from the 3.0 (N2 & N3) that attach to the body wiring. You can purchase one from Offroad Solutions or make your own if you're savvy with electronics. Either way, you need the correct EWD (electrical wiring diagram) for the motor and it would help to have the EWD for your 91 truck as well. Contrary to what others may say, you only need the 3.4 engine wiring harness and ECU, and the 2 other plugs that attach the ECU to the vehicle wiring. (no need for the entire under dash wiring harness) There is quite a bit of work to make this as you need to basically cut & splice every wire going to the ECU to do this properly. You will want to solder and heat shrink every connection if you want it to last.
Regarding the gauges- the temp gauge sender is the same resistance range as the 3.0 so its a direct hook up. the 3.0 oil pressure sender will screw in to the 3.4 block but needs to be extended slightly to clear the alternator. The tach will not work without an adaptor. I can supply you with the oil pressure sending unit extender- $10.00 and the tach adaptor- $45.00. I had to build a tach adaptor as no such beast exists to my knowledge.
The bellhousing will bolt right up to the new motor as well as the 3.0 motormounts- However, the right side motormount may only hit 2 or 3 bolts on the block depending on the casting and whether or not you use the 3.4 A/C bracket. I used the 3.4 A/C bracket and compressor and swapped the top of the compressor from the the 3.0L so the hoses bolt right up. Use the 3.4 flywheel, clutch, pressure plate & pilot bearing, Use the 3.0 release bearing. You will also need to cut the P/S banjo bolt down from the 3.4 to fit your 3.0 high pressure P/S line.
Other issues- use the 3.4 alternator and splice the 3 wires into the original 3.0 harness since the plugs are probably different. Re-route the fuel lines and evap lines. Cut a hole in the hood and use a scoop or go with a 2" body lift. I did a scoop because I don't like body lifts. My scoop is at the following link:
http://www.idmspeed.com/sr6pic01.htmlExhaust is the biggest expense / amount of work- I used Downey Offroad headers for the 3.4 manual transmission and cut them to clear my frame. Then I fabbed it to run past the xfer case and merge just behind it. It's a ***** to get the sensors, cat, muffler all in a short span before the tailpipe. Cost me around $700 for the headers, cat, muffler and labor. I can supply you with the sensor mounts- $10.00 pair. Mike at ORS may have a crossover pipe that will adapt the 3.4 manifolds to the 3.0 routing (left side). This would save you alot of time & money.
The end result is bitchin though! I can scream through the 1/4 mile in just under 15 seconds! (I've got the supercharger)
