I have personally used T61 palmrest on T400 and even though the threaded mount of one screw had to be filed down a bit, it worked perfectly. T61 has a bit smaller touchpad also.
Yes, I'm aware of the longer "stand off" on one of them. Shortening it to make it fit is an option, but swapping the other direction would require lengthening it, or removing it altogether. Leaving it without a screw should be avoided because if someone ever did install a screw, it would likely break the part. I believe it was richk that alerted me to this.
I do know you can swap LCD screens, assuming the T400 isn't led backlit, and it might be possible to use a t400 cable in your t61 lid to get past the systemboard connector problem, but unless you have a pile of parts and want to do this for entertainment, I suggest it's a very bad idea.
However, we'd all love to know if it can be done.
ps. considering I just sold a really nice high-end t400 for less then $300, I can it worth all this effort to end up with something less then a t400 in the end. If there was some unique objective to achieve, then I'd say go for it, but if you're only trying to fix a dead system board, then I'd just get a t61 intel board and be done with it.