Brazing stainless for cryo temps?

I want to make small closed-type impellors for pumping cryogenic liquids (LOX, perhaps later LH2) under high stress, low weight conditions. This is for experimental purposes, at least at first. Suggestions for a production version are also welcome.

It's part of my ongoing effort to make turbopumps for bipropellant cryoliquid model rocket engines. :) I'll probably use mini-jet turbine parts for the turbine wheels.

The impellor has complex interior surfaces, so I thought of making it up from computer generated plasma/laser (?) cut ~1 mm layers of 304 (?) stainless. I'd put removable tabs on the outside of each layer in order to get the registration right, and bulk braze them together to form a whole. Then heat-soak for a while, partly to distribute stresses and partly to allow the braze and the base metal to mingle.

Any suggestions for a brazing material?

I'd then like to coat all the interior surfaces in a lower melting braze, in order to smooth the interior surfaces - basically two-step brazing, except the second stage is just a covering/smoothing layer.

Again I'm looking for suggestions for a brazing material. I thought of using

449 and 456 (449 and 456 are UK types), any comments? Any sources for 449 in small quantities?

Any other suggestions or comments?

The finished impellors will have to pump LOX at cryogenic temperatures.

Thanks,