I have a 727 that I plan on putting in my car this winter. Been cleaning it up as it was pretty dirty/greasy. I got most of the crud off with various cleaners, steel wool and Scotchbrite pads. I can't get in all of the little nooks and crannies though so I was thinking of hitting them with my hand-held sand blaster. Of course I will have all of the areas where sand could get in the trans. taped off. My question is, will the sand damage the aluminum? If so, maybe I could lower the air pressure to a safer level?. Also, is there a way to clean off some minor rust on the access cover that bolts on to the bottom front of the trans. that will not ruin the galvanized look it has?

I know this isn't an answer to your question, but couldn't you get those nooks and crannies with brake parts cleaner?

They make aluminum etching chemicals, a mild version is sold for uncoated aluminum wheels.Find someone with a high powered hosty or google and youtube

Sand will change texture of the case. Find a soda blasting place. Or vapor blast with soda. Warm water rinse is all that is needed afterwards.

Take it to a good tranny or automotive machine shop and have them clean it in the spray washer or dip tank
Make sure it is good and clean and dry

Abrasive cleaning and precision assemblies don't play well together.

What's your main goal to have it clean or bone stock looking? Some tanks don't clean all that well because the fluid hasn't been changed. I have had blocks cooked that still had grease on them because of this. Tape doesn't always keep out blasting media, I found this out the hard way. What media you use will effect the surface look of the aluminum case(or other metals). Again it depends on the look you are after. If you are cleaning it to paint it the finish doesn't matter all that much.

Spray Nine works pretty good on aluminum. Spray it then scrub and let is soak rinse and repeat. Small wire brush for the nooks. I have never tried the acid type cleaners.

Hot aluminum-safe detergent does a great job with no surface alteration.

I have used spray on paint stripper with very good results.

Had the same issue last week. For the nooks and crannies, I used a STIFF rotary wire brush; the type with the wires protruding straight out of the end parallel with the 1/4" mandrel. I put it in a cordless drill and you can force it into all the corners and nooks, etc. Took 10-15 minutes and worked great and didn't affect the appearance of the surface overall. The brush I used is worn down to about a 1/2" at the tip so it fit those areas pretty well.

I've had semi good luck with a cheap siphon feed blast handle, a short piece of tubing push into a box of baking soda. Have to shake the box, but can make a carb (Carter) look new. Also good for electrical contacts.
A filter mask is needed, as the soda doesn't taste very good.

I used paint thinner and an assortment of wire brushes.

My friend had a Transmission Washer. You put in a scoop of cleaner and it spun it and washed it. When he shut his place down he gave it to the Machine Shop that does his engine work. They used to bring him Aluminum Blocks works great.
Yes sand-and media will change the texture. So will the 3-M abrasive disc guys like to clean stuff with. And sand paper will 100%.
I am thinking of making a Lamp out of a Tail Shaft

and of course the tail shaft housing will be polished so good, it will look like it has been chrome plated ! [if i know how you do things. ]
please post a picture or two when you get it done.