One thing to consider is that if you go the Toyota 5.7 route, the tcase that comes with the 5.7 has a driver side drop. IIRC, the T100 has a passenger side drop, and I would assume your d44 has the pumpkin on the right side as well. The 5.7 is a pretty new engine, and I would be very surprised if anybody has a kit to mate it to the tranny you have already. And I doubt the A340 (if you have an auto) could handle that kind of power over a long period of time anyways. Pretty much anything can be done with enough time and money, so I'm not saying a 5.7 swap is impossible, but it would require alot of work.
So from the little bit I know, I think the only way you could get a 5.7 in there is to either buy a 5.7 from a 4wd Tundra and swap tranny, tcase. and get a front axle with pumpkin on left side, OR buy a 2wd Tundra 5.7 and run a divorced tcase with the drop on the passenger side using your D44 that you have already, OR build a custom bellhousing or adapter plate and try to see how long your A340 will hold up to the power. And none of those options even include wiring, which I have no clue about.
Personally, if I was going to do a engine swap in a T100, I just drop in a 4.7 and be done with it. It'll be a bit of work, but easier than a 5.7 and you still have a Toyota v8 motor.
Or you could just do what Kevin suggested and drop in a SBC.
just my $.02.