2004R has better gearing in it! I put a 2004R in my 69 with a 400 and it is a direct fit for the most part and you can use your same shift linkage (assumming you have consol shift) But you will need to get over drive conversion kit from SHIFTWORKS and you will also have to remove your Neutral safety Rod that hooks to your tranny and runs up to your steering column and get a NSS that mounts to the shifter in the console from shiftworks. Yes, you should be able to use the TH400 crossmember. I didnt because I had some problems and ended up buying a different one to work. Also, if you have a driveshaft from the TH400, you will have to cut it down slightly, the ones for the TH350 should fit from what I have read. My tranny coolent lines from my TH400 would not work on my 2004R, without a lot of bending and such, so I went with AN6 flexable steelbraided line and used an external cooler, which is recommended, which I would highly suggest that you do. If you are looking to buy a tranny thats already set up, a lot of companies sell them and beefed up slightly to highly beefed up to handle the power, and if your going to build that 400 up to around 400HP, then you will need at least a stage 2, 2004R. I love the 2004R in my car, and I have a 8.5 rearend in it with 3.73 gears and with the TH400 in it I was screaming at 55MPH, now it purrs down the road! Got mine through Gearstar.