As someone else who is swapping in a 3.6 variety engine, I went through many of the same questions you did.
If you go V6 go with the LFX 3.6, it has most all the mechanical bugs worked out but without DOD and other GM efficiency electronics of the LGX.
As plentiful and powerful as the 3.6 is...and it is powerful.. they make almost full torque by 2000-2500rpms and maintain it out past the hp peak with the DI and VVT technologies......just overlay the Gen1.5 Acura NSX dyno with that of a tuned LFX... they are almost the same with the LFX making more horsepower and torque and without revving out to 8K. Having said that it "is" GM's disposable workhorse engine they throw into everything with little aftermarket support... much like the 2.8-3.4 60deg V6s of the 80s and 90s.
Your power will be limited in the 3.6 to tunes and expensive cams for not 'that' much more power. The 3.6 cannot take much boost as it was designed to be the 'most' you could pull out of a N/A engine with VVT and DI. Unless you go full rebuild or LP9/LF3/LF4/LAU but then $$$$$, I know.....
You are limited to the factory ECU due to the DI setup unless you pay big $$$$ for a Motec... but given the complexity and engineering GM put into the maps with VVT and DI no real reason to abandon them.
You will have to deal with 'two' fuel pumps, the in-tank one and the mechanical High Pressure one... your in tank fuel pump will have to support PWM plus greater output.
Be careful which 3.6 you grab... some have the Fuel Pressure Control Module integrated into the BCM and some separate. Go with the separate.
The miata and RX8 crowd have been swapping out LFX's for a few years and can be utilized for knowledge as well.
You have a quick axle solution with the cobalt axles but you have to offset the engine 2" toward the PS. If you are going to rail on the engine, the cobalt SS axles are probably not going to hold up... then its custom.
Your transmission is limited to the auto that comes with the 3.6 if transverse or running a Saab 9-3 F40 setup. Mender on the board here has adapted an F23 to the 3.6 with a fair amount of bracketry.
If you are dead set on a 3.6 but also will want more power, do take a gander at the LP9 out of the Saab 9-3 Aero or 2010-2011 Cadillac SRX. It is the same block as the 3.6's but 2.8L (can be bored / stroked) but with no DI or VVTi, 280hp stock with a vicious torque curve and you have all the go fast turbo bits internally and included for upgrading. The only downside is the bosch ecu in the earlier saab 9-3's but I 'think' this has been dealt with in the saab community. The cadillac SRX's aren't known to have this option by many so you can get a mildly driven 'caddy' driven one of these.
The LS4, although is a bastard child of the LS family, it still is a relative which has large amount of aftermarket support...LARGE.... so thereby inherits some of this...don't EVER underestimate aftermarket support..
The LS4 is compact height-wise but is still a V8 when it comes to width. The 3.6's are narrower but a fair amount taller due to the DOHC heads, not quite LQ1 large though. You will have a 'tight' fit between the towers with an LS4... The 3.6s being very tight against the firewalls.
The LS4 with cross GM LS modifications/parts, see FieroGuru, will give you ~340whp... that's NA wheel horsepower with only a mild cam and great torque curve.... that is damn impressive with the simplicity of no VVT or DI.. A valve train upgrade on the LS4 will allow it to rev to 7000. The 3.6 with all the NA goodies available.. i.e. $$$ can barely break into the 300whp range and you have to rev it out with those mods... ultimately an engine is an air-pump.... the LS4 will always win out as it simply has more displacement with just as solid engineering as the 3.6 V6s.
The LS4 has the same custom axle issues as the 3.6s more or less.
Factory GM ECU or any aftermarket ECU can drive the LS4.
Mounting-wise the 3.6 fits relatively well between the front and rear cradle rails while the LS4 can require modifications.
Both will require custom mounts throughout with pre-made LS4 ones available through WestCoastFiero.... ymmv though..
My personal thoughts....... if you enjoy winding out your engines a little more go with the LFX 3.6.. it's like a more complex more mature LQ1. If you enjoy the sound and early torque of a V8 go LS4. Both would be quite drivable and enjoyable in stock or mildly modified forms.
My aggregate research results in terms of hp goals vs cost between the LS4 and the 3.6 HPV6s:
250-300 - LFX (shear availability and generally lower mileage availability in yards)
300-400 - LS4
400-550 - LS4 or LF3/LF4
600+ - LS4 (this is where the LS aftermarket really comes into play)
Last thought... the 3.6's sound like crap..nothing like the 60deg V6's of the 80s and 90s and I don't know why yet...
I've only heard a couple of decent sounding 3.6's a Camaro LFX with a Borla and an ATS-V with Borla... The LS4 well........because V8...
if only they made a flat plane 4.8L destroked crank for it...
[This message has been edited by msweldon (edited 01-10-2021).]