Trans: no TKO-600's in stock applications. Closest thing might be a Tremec 3550? used in mustang cobras, i think. Not many used TKO-500 or 600 out there yet so new is really your only choice.Mine is Ford application #5008 w/ 0.64 overdrive ratio. A 0.8X ratio is also available just make sure you get the one for a Ford application since the GM version has a much shorter input shaft and since mopar used the longer input shaft than Ford or GM it really help having the added length of the ford shaft.
Throwout bearing:I found this bearing while installing a clutch in a '97 4.0 wrangler. it is Napa # N4093. also used in many later dodge trucks too.
Pilot bearing: 2 parts are required. One is Mopar # 4338876 and the other is Napa # B65174('95 Mustang GT application, i believe) Both parts are 2 piece. Use the bearing (inner piece from B65174 and outer piece from Mopar 4338876) press bearing from B65174 into housing from 4338876. It will be very clear once you have these in hand. used a big washer(1/8" thick) to space these forward to ensure that the bearing has maximum contact with the tremec pilot bearing surface on the input shaft.
BTW the whole pilot bearing should cost much less than $20
trans mount:I threw the box away but is was the same as Energy Suspension # 355-3-1108G (Jegs number since I have a Jeg's catalog handy) I've seen this same mount at AutoZone. I'm no GM expert but I think any parts store mount from any GM rear wheel drive car from the past 25-30 years is the same as the Energy Suspension mount if you don't want to spend $20+ for the Energy Suspension mount.
trans slip yoke: 31-spline ford found on C6, toploader, some AOD's. If converting from a 904 auto or a 4-spd A-body you will need to shorten driveshaft approximately 1 inch. It is up to you how you want to handle the driveshaft. Slip yoke most likely will use a spicer 1310 or spicer 1330 u-joint. Mopar Driveshaft will have either a Detroit 7260 or a Detroit 7290 u-joint. Do whatever you want to connect these together. There might be a conversion u-joint for your combination or if your slip yoke uses a 1330 u-joint you might want to get the weld yoke on the mopar driveshaft change to a Spicer 1330 style so that you have a strong and commonly available u-joint. If in doubt, talk to your local driveshaft shop. get conversion u-joints at you local napa or spicer driveshaft shop.
crossmember: you are correct, you need to make one. just make sure you keep the front and rear driveshaft angles within specs. Pretty easy to do on an A-body since the transmissions are within 1" of eachother in length.
Bellhousing: A stock bellhousing would require an adaptor plate which means $$$ machine work. A plus is that scattershield are relatively low cost when bought used and keep the trans as far forward as possible eliminating further problems with the pilot bearing and keeping the trans mount in position at the same time. Just by luck the trans mount is very close to the stock location in an a-body when using a scattershield and you really need to keep the trans as far forward as possible since a Ford input shaft is already shorter than a Mopar shaft.
Clutch disc: If you converting a mopar with 10.5 clutch simply get a 10.5 GM disc with 26-spline hub. 11" mopar get 11" Gm with 26 spline hub. If you have a high $$ Ram, McLeod disc talk to them... they might be able to put a 26 spline hub in it for you for much less than a new disc.
Pressure plate: whatever you are using now. The throwout bearing # that I posted ought to work with any style pressure plate i.e. diaphragm(centerforce), O.E. mopar style Borg&Beck or Long style.
Clutch arm: Stock mopar
Clutch linkage stock mopar.
I just need to put a mopar end on a Mustang speedo cable. I've been putting this off since I started driving the car with the Tremec installed over 6 months ago.
NOTE: a lot of this stuff becomes really obvious one you have parts in hand and begin the wrench turning. I will try to post some better AND LARGER pictures in the future hopefully with the car on a lift.Like I said before, the only work you need to farm out it turning the front bearing retainer down to mopar size(Jerry posted above that he will do it for $50) and getting the driveshaft shortened(forgot to include that above). also to fit a box stock tremec in an A-body you must cut out the loop above the trans mount but I really can't comment on B/E bodies.