Yes, keep the diff, assuming it is a T series. If it has coil springs in the rear it will be a T. Keep the tailshaft as you will want the front and rear yokes.
You may get away without having to narrow the diff as the TA22 T series is only marginally wider than a TE27 T series, as long as you don't go too wide or silly with offset..
You will however need to remove the spring seats and have leaf spring mounts added, so keep your KE25 diff and cut the spring seats off that. If you leave the panhard rod mount on the diff, you could try welding a mount to the chassis so you can add a panhard rod!
Obvious advantage is that you can bolt in a LSD from an AE86 which are readily available. And there are various diff ratios available. I believe TE27's used a 4.3 which most T series AE86's ran standard so your tacho dash speedo will read properly! Just make sure you get an earlier Zenki centre, as the axle sizes got bigger in the later kouki diffs.
I have a TE27 tailshaft here, plus a TE27 axle housing on the way, so if you need specific measurements, happy to take these for you.
If you get real excited, there are rear disc conversions via Toymods that don't look too hard.