I hope I am not repeating a too-oft posted question, but I have seen people talk about putting "V8 guts" in their slant 6 904 transmission. So I have to ask, what are "V8 guts"?

And a related question: How would a guy prep a /6 904 for street/strip with about 240 HP?

Drums and clutch packs that hold more clutches.
Could do a stock rebuild, could put in V-8 guts, could put in a shift kit, could put in a reverse manual shift valve body… What's your preference?

Reed, when you say "drums and clutch packs that hold more clutches" are you talking about getting another transmission, out of a V8 car, and transferring those parts into a /6 tranny? Or do you mean that the rebuilder puts these things in and they are readily available?

As for my preferences, I want a transmission that will still shift itself in Drive, but can be effectively manually shifted as well with my B&M StarShifter. I want firm, but not tire barking, shifts. In other words, tough and firm, but streetable.

Both. "V-8 guts" are the internals from a V-8 904 transmission. If you compare specifrications, you will see that over the years ther were varieties of 904 transmissions produced. 904s for six cylinder vehicles for the most part had lighter-duty components than transmissions built for v-8 service. However, nearly all of the internal parts can be interchanged between six and eight cylinder transmissions.

I encourage you to search the posts on this site to find numerous discussions about swapping parts to improve the performance, longevity, and efficiency of the 904. THis has all been discussed many times before.
You can manually shift a stock 904 (1,2, then D) as well as let it shift itself in D. My recommendation to you would be a Transgo SK-1 shift kit.

Thanks, Reed.