I don't think it will be as much of a problem as you might think for you to spool an HX40 with your displacement. HX35 with 12 cm^2 turbine starts boosting by low-3000 rpms on a 1.8 liter Honduh, with 10 psi by 4200 rpms. It seems to me like it would be a little restrictive on a V8, but I remember scuttlebutt about a Mustang somewhere along the lines that was running low 11's off a single HX35.

As far as HX40 sourcing, I'm a big fan of starting with www.btnturbo.com and their online turbo catalog... sure, it's euro
stuff, but a goodly portion transfers overseas. Enough to give you the tips you need. Going through the Dodge/Cummins boys you'll end up dropping $$$ via piersdiesel or the like, as the HX40 is in much demand.

I think I'd like to add a disclaimer at this point. Keep in mind, I
play with slow small displacement japcrap, and have no experience with any of these turbos on large displacement domestics, and only experience with the HX35 on one Honduh. Keep your nose to the ground, see if you can scare up anything more substantial than what I say.

Someone made a passing comment once that Turbonetics had a very good grasp of the Holsets - you might want to give them a call.