That $750 rebuild is going to be much more appropriate. You don't need to be thinking about special stall speeds. Get a stock rebuilt torque converter; that's what's suitable for the kind of car you're building. 3000 RPM is much, much too high; that would be a race car piece, and you'd be pissing away gasoline money every time you drive the car, and heating up the trans fluid to the detriment of everything, and it'd be a poor match for the low-RPM torque motor you'll want in a fun-driving car like you're building. Attached to this post is the factory '69 stall speed chart. Your situation is the 2nd line in the chart: 225 engine and 904 transmission. More/higher is not necessarily better!
And be very picky and specific with the shop about that "mild shift kit". Specify a Transgo TF-SC. You could go one step further to a TF-1, but there wouldn't really be a benefit and you might not like it so much.
There really isn't a whole lot of re-engineering or upgrading needed here; you're starting with a very well engineered transmission.
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