As the Brits say, they're alike as chalk and cheese.
The BIP is 120x120x20(or so) - frontal area 144 sq cm
The 2-342 is 240x156x50 - frontal area 374 sq cm
IMHO, frontal area is the most important dimension - and the BIP is only 38% of the 2-342. I'm sure you do lose some heat transfer efficiency as well - what with the BIP less than half the thickness of the 2-342.
Maybe think about the BI II (120x240x~20) or 288 sq cm - and up to a whopping 77% of the 2-342...?
Or maybe dig through the heater core database.
Or maybe use something other than an axial fan. I've been trying different impellers - so far, for equivalent air pressure/flow, I haven't been able to get them any quieter than "stacked" undervolted axial fans. Please don't let my failed experiments slow you down though - if you come up with something, I'd love to hear about it...

I did mean to say BIP 2, since they're both 2x120mm. Sorry about that. Luckily, I might be buying used 2-342 already painted and what not for about $30 shipped.

I'd really like to know what BillA is hinting at that we should use but not saying.

BTW, I checked out some motorcycle rads (someone mentioned 'em somewhere), and they're just not right for the job. Many are contoured to the shape of the bike, the inlet/outlets are usually pointed in all different directions and many of them have less surface area than a heatercore. And to top it off they're more expensive. I thought I might be able to get one for cheap, but you're just not gonna do it for less than a heatercore.

should be apparent from Swiftech's offerings, 2 thin rads and the 80mm thick rad which is all that exists
thin is more efficient than thick, thick only being superior with SHE fans and above

so what I have not said is that every user of a heater core with less than a SHE fan has a typical DIY setup,
cheap parts that work ok
- focus on cheap, not performance (unless they have those SHE or EHE fans)

not my way at all

efficient?

in a given range of air pressure, a thinner radiator will have superior heat dissipation when compared to a thicker radiator of the same basic design

(thats what I think you are saying)

I'll just assume you've tested this enough and not nit pick.

And where does fin density play into all this? Does have as much impact as the depth of the radiator?

(I assume this is why Swiftech's rad is so thin)

at this moment I'm not inclined to say much, but do read Cathar's comments and it is all pretty clear

I'll have to buy a Swiftech 2x120mm rad and compare the performance to the 2-342 I'm getting. Being that I'm using 120mm Sanyo Denki's, I'm not really sure how it'll turn out since they're in the middle as far as 120mm fans go. More powerful than most Panaflo fans. The H1A matches it for CFM (103 Panaflo, 120 Sanyo) and mmH20 (6.8 Panaflo, 6.6 Sanyo) but the H1A is rated a full 3dba louder (however, Panaflo is a tad optimistic in their ratings so it's probably more like 5dba). The Sanyo's surely no Delta of any sort though. Would you happen to know what the Swiftech rad was tested with?