I am not a trans guru, but I have built my own for a 600 hp application. And I have torn them down after a years running and inspected things to see how they held up (twice). Also, I have done a few for friends making less power. So, my 2-cents to get things started:

1) Get a good valve body
2) Get a good valve body
3) Use a 727, they are strong as an ox

- a heavy duty front band servo strut is available from a few vendors, Superior #K031 for example.
- Sonnex makes a billet rear servo piston that doesn't crack, and resists getting cocked in the bore. #22912
- for low band apply valve bodies, I think a stronger rear servo retainer is a good idea, and shim inside the small piston spring close to solid, so the cushion spring doesn't compress.
- use the '70 and earlier front servo (not the '71 oil cushioned)
- Set the clutch pack clearances on the tight end of tolerances, especially the rear clutch
- Get some new bushings put in almost everywhere if it hasn't been done in 30 years.
- The front pump bushing (for the converter) is a mandatory replacement everytime, use the babbit ones
- A deep sump pan and filter extension is a good idea and makes the fluid level less critical
- New springs and rollers in the sprag ($2), or a new bolt-in one

See my trans page on my website for some other options:
440 Jim's web site - 727 Trans Tips