If you size the turbos correctly you can get more mass flow out of compunds for a given boost # than you can out of a single.
You almost never hear about that because most people with compund turbos are going for max power.
I've seen some of the diesel guys go compunds and pick up both power and economy due to the lower "drive pressure" (back pressure in the exhaust between the engine and turbos if you prefer).
They seem to go there for both the broader torque range (earlier spool without extra foolishness, like you say you want) and lower EGTs (more air in a street diesel = cooler EGTs as they run lean of peak, all the time).
Charge cooling before the mixer isn't a bad idea, as you definetly don't want it after the mixer.
(yes I figure you already know that, but it bears saying)
The hard part will be sizing the turbos without spending a fortune.
An external wastegate for the high pressure turbo seems to help on the diesels as they tend to have the high pressure turbo's turbine turn into a choke point, to the extent the low pressure turbo can have a hard time spooling.
I like the JGS brand, but I would, they're made domestically, not at all like the $bay chi com stuff, so kinda pricey.
For whatever it's worth I've got a 3,9L (so ~ 239 CID) Diesel (Isuzu 4BD2-TC 3200 RPM redline) with a HY35W Holset single on it.
I have to put the injection pump back on (had it repaired) but the HY35 seems to spool somewhat late. It's the stock turbo off a Dodge Cummins, 5.9L, with that automatic from a certain year range. I wouldn't normally try that much larger a turbo, but the Hy was undersized for the 5.9 to keep the smoke down as the converter slipped in lower gears.
I've been looking at building a compound set for the diesel I have (see, a tie in, crazy aint it?
).
The two combos I have found discused (ie people have tried) are a HX30W Holset (smaller than what I have on there now) for high pressure, with a HX40 variant for the "low pressure" turbo (basically what Cummins, Cat, etc used on the 8+ liter engines, ala Kenworth, etc). That seems to be the prefered setup for a stock engine with an unmolested injection pump and stock injectors.
The other, that seems to be prefered by people with lighter vehicles, and modified engines / injection stuff is the HY35W or HE341 (very simliar size, '341 might spool a little quicker, and move more air as it's newer tech) over either a HX52 or a HX55 Holset. Both are huge and people seem to like them, when they want to make a 100-120 HP engine into a 300- 400 HP engine (I would be delighted to get 225-250 out of mine, it started at roughly 150, these are diesel #'s though).
So that wouldn't likely translate directly, as you have a somewhat larger engine (4.9L) and faster redline, but lower air requirement per HP (mine runs lean of 22:1 or I'm pouring fuel [black smoke] out the pipe).
The Holsets and Borg Warners (S200, S300, S400 series) are available as take offs, from people that are either upgrading or otherwise repowering, often for fairly cheap. It's why I posted all this, I thought you might want to know.
Hope some of it helps.