Flintlock76
Continuing the research I hit every steam book I have here in the archives of the "Fortress Flintlock" and can't find anything to nail down where that "Zeppelin" name came from.
See the ad on p.57 of Tom Murray's book "Chicago and North Western Railway" (accessible via Google Books in the States if you don't own a copy) and then pull up the Complete Collection to read Wallace Abbey's article in October 1970 Trains (" ... Zeppelins they surely were not"). I dimly remember an article titled "An H of an engine" but nobody aside from TrainOrders even mentions it on the Web.
And just for the record, it's still the Fortress Firelock, unless Her Honor now goes by "Lady Flintstorm" which loses something ineluctable in its semantics. Just because Clambake fudged your login credentials is no reason to tamper with the admirable.
Frankly it would be interesting to see if you could shoehorn this into PRR clearances for a WPB choice (using the blueprints for the first rebuild, which changed the H from merely awesome to near-perfect 'for purpose') with that Jones1945 patent semilune-windowed cab (and perhaps a rightsized Belpaire box and chamber, perhaps of Q2 derivation, if you can finagle it). Go ahead and spec as lightweight a set of rods etc. as you can since you're building it new.
Control dimension is 100" max boiler diameter (plus cleading/lagging) over 76" drivers ... this rings a bell for you Double-Belpaire fans, nicht wahr?) for a nominal loading-gage height of 16' even ... which I think is still a tad high to get all the way into Chicago without a fuss. But look how close it would come to a practical fulfillment of the original Q1 design parameters!
And yes, it's a pity the 1946 rebuilding specs wouldn't have been available for that purpose, as they essentially made this package what some gamers call god-level. Read about the H1s in the contemporary trade press if you want to see something impressive (and unaccountably underrated). It was as much a crime not to have preserved one of those as it was to lose the Niagaras. And yes, that's a strong statement.