I found JayBee's post, here it is
Ok, I am still getting occasional requests for info on how to do this swap, so I'll make a post with all the info I can remember on how to do it. It's been almost 2 years since I did it, so please excuse anything I forget to mention.
I now have about 20,000 miles on the truck with the conversion, and could not be happier. My only complaint in that time was with the ceramic puck style clutch disk I used. I have since replaced it with a conventional LUK disk and love it! There will be more on this to follow.
First of all, you have to remove the approximately 1" thick adapter plate the IDI's use in order to get the clutch and flywheel spacing correct. In other words, the bellhousing on the ZF6 is deeper and the input shaft is shorter, so the spacer has to go. The 7.3 powerstrokes have a different bellhousing bolt pattern cast into the block compared to the IDI. The good news is the 2 dowel pins, and 4 lower bolts do match up. The upper 2 bolts are the only difference. I made a stepped adapter to go from the upper 2 bolts on the IDI block to the upper 2 bolts on the ZF6. It was 2 pieces of 1/2" x 2" flat steel bar overlapped and welded together, making sure both bars were parallel after welding. The lower bar bolts to the upper holes in the block, and the upper bar sits behind it and has the upper 2 holes tapped for the tranny bolts. That makes the front face of the upper part of the adapter flush with the rest of the bellhousing in the block. I bought the stock tranny dust plate for a '99 and had to cut a little clearance into it for the adapter.
The next big hurdle is the clutch. The ZF6 has a 1.375" input shaft, and the smallest disk you can find with that spline is going to be 13" diameter. A 13" disk will not fit under any pressure plate that fits an IDI flywheel. My first way around this was to use a custom made 12.25" disk with the 1.375" spline and 4 ceramic pucks per side. This disk turned out to be miserable! The more you tried to slip it, the harder it grabbed sending the truck into convulsions. Backing up to a trailer was virtually impossible. So, I recently went a different route.
I had already used a LUK solid flywheel, so I pulled it back out and redrilled it for a LUK 13" PSD pressure plate. I now have the LUK pressure plate and disk for a '99 PSD and it works great. It reduced the pedal effort, virtually eliminated the gear rollover noise, and is very smooth. This is definitely the way to go. Oh, and you can't use a LUK flywheel for a PSD because they have a different bolt pattern on the crankshaft.
The clutch hydraulics weren't too bad, but not a simple bolt together deal, either. I used the correct master cylinder for my year truck. I used a slave cylinder for a '99 ZF6. The fork, pivot, and throwout bearing were all for a ZF6. The only real problem was trying to come up with a clutch hose that would work. I ended up making adapters that fit into the clutch and slave with a -3 AN on the other end. I then made a stainless steel braided hose to connect them. Bleeding was the typical pain, but it has worked great ever since.
The shifter ended up in exactly the same spot as a ZF5, so the tranny tunnel cover for a manual and boot all bolted up fine. I made my own cross member for the back of the tranny by heavily modifying the one that was in my truck with the E4OD. You might be able to bolt in a cross member from a '99 ZF6 - I'm not sure - was too cheap to buy one.
Of course you'll have to visit a driveshaft shop and get yours modified or a new one made. I also went to a '99 E350 disc brake rear axle at the same time, and had an all new driveshaft built to fit.
The ZF6 does not have a speedo drive gear. This is where a '92 up truck has the advantage. I used a speedo from the '92 up truck and made it fit inside my cluster with some slight customization. The '92 up speedo is all electronic, and I was able to drive it from the sensor on the rear axle.
The 7.3 IDI starter bolts directly into the bellhousing of the ZF6 and works fine. The bellhousing lower cover for the '99 bolted right up.
Oh, I did have to bend the floor of the truck up just a little bit for clearance, but it's under the seat and totally unnoticable.
I think that about covers it... If I remember anything else I'll reply to this post. If anyone wants a flywheel redrilled I could probably take care of that part of the conversion for them.
Jason
I do believe i will be able to pull this off. Once i get the tranny out (next summer) i can see what was done under there and do this mod to it. I like the idea of using stock parts rather than tractor parts. And i have a machinist that can pull that off as well.
As for the adaptor for the top two bolts, hell im sitting in a full blown blacksmith shop. i think we can pull that off without much trouble.
For the clutch line, well, he ran the line against the exhaust pipe from the manifolds all the way back, so i planned on rebuilding that setup anyways to bring it under the cab and over. so i should be good.
Sounds like for me it won't be such a huge deal to fix it right. Then get a new drive shaft of course.
, could be. right now my money is he measured the shaft with the front wheels in the air. then when it was on all 4's, it was too short. 
