i have a HX sandman thats factory M21 4 speed, and i will be running a T400as i love turbo boxes, + and i don't think it really affects value.
its a 4 to 5 hour job to convert it back to original provided everthing is on hand and ready to go. + 2 hours if you dont have a hoist!

in saying that a would think less of a Sandman with 253 + trimatic than 253 / 4 speed

but i would rather a 308 / T400 over a 308 / 4 speed