(originally posted 2013,updated 2016)Hello i have been a lurker on this forum for years and well i did a 3400 swap in my 99 Monte back in July of 2012, I wanted to share what i learned during the process,also i manage a salvage yard and have been a Dismantler for years and had the ability to cross reference/compare everything i needed for the swap as well as pick the perfect engine for the swap. I used a 3400 "LA1" out of a 2003 Pontiac Grand am w/o secondary air injection, we dubbed the project the "Montiac".

As most people already know the 3100 and 3400 blocks are near identical. It is the bore size that is the difference,so the engines themselves will drop right in,The 3100 bore size being 89mm with a 84mm stroke.The 3400 has a 92mm bore and a 84mm stroke.
Just make sure you get a 3400 from a vehicle that did not have secondary air injection or the heads will be different..Generally a federal emissions car will not have it versus a California emissions vehicle. I went with an 2003 because GM had the piston slap and intake leaks worked out on these engines by around 2002.

So for this swap i used the stock 3100 PCM which ran my 3400 with no issues drive-ability wise. i am even able to pass New York state inspection which requires no that there is no check engine light on or trouble codes stored in the PCM to pass the annual state inspection,

(2016 update: i have run the car for three years on the stock PCM. Only now years later i have decided to finally send my pcm out for a tune now that its no longer my only vehicle.)

So, for the main parts of the swap You will use the the 3400 Engine block,cylinder heads,intake manifold,throttle body,Egr valve,and exhaust manifolds,

All other accessories will have to be from the 3100 engine. In my case the oil pan (had a sensor), alternator bracket,Alternator,a/c compressor and bracket, p/s pump ,stock fuel injectors w/rail, Coils (both are the same) ,cam sensor,oxygen sensor,steel heater lines,engine mounts,wiring harness,vacuum lines and any other misc. sensors and brackets.

(p/s pump and a/c compressor were never fully removed from the car,they were just unbolted and put aside out of the way with the lines still attached)

valve covers are the same.

You will not need to remove the timing cover/water pump/front cover at all..

Stock 4T60-E transmission as well

The oil pan had to be swapped in my case due to the Grand am 3400 engine did not have a oil level sensor mounted on the bottom of the oil pan . I know some other 3400's from other vehicles did have this sensor in the pan which will eliminate the need to swap the pan.

In my case The cam sensors are different..the 3100 has 6" of wire with the connector at the end, on the Grand am 3400 the harness plugs directly into the sensor and is a smaller connector, It is easily swapped just remove the bolt and slide the sensor out but be careful not to break the retaining bolt or break the 3100 sensor during removal. again some 3400's will have the correct cam sensor and will not need to be changed. it all depends on the donor car..

use the stock (19#/hr) fuel injectors and rail from your car, Injectors are designed to work with your PCM tune and cannot be swapped for 2000+ 3400 injectors (22.5/hr#) without wiring in the different connectors and having your PCM tuned. The stock 3100 fuel injection rail itself is vehcle specific so the fuel line configuration will be different from any other car so i used my stock one. It all fits into the 3400 intake manifolds. Its a tight fit, but it fits..The upper plenum will push it down ever so slightly on the rail when you torque it down but it is a non issue for me, i watched it carefully..

This is a strait forward drop in swap, after you swap all the accessories and Replace necessary gaskets the engine drops in nicely and bolts right up.

While I Had the oil pan off I went ahead and replaced the oil pump with a high volume oil pump,They are available everywhere i shopped around and i got one from Napa for about $100.it was a little insurance to help me feel comfortable beating this engine up with no isses

The 3100 Ecm runs the 3400 just fine with limited drive-ability issues,all i have noticed is a slight misfire at low speed/low Rpms which is remedied by more throttle and I'm not 100% sure that that is the ECM causing the misfire but I'm 90% lol. I'm working on it..granted i may not be getting the most from the cubic inches and an Ecm flash is in its future..But i am vouching for the stock tune making this 3400 motor scream like a raped ape.and with more torque and high end than any other 3400 car i have ever driven. Its just down right nasty at 4500 to 5000 rpms lol

well that's all i got for now, I hope this helps someone.i have a few pictures up in my profile here but not many i tend to get wrenching and forget to take pics. But If your experienced enough to change an engine then you can handle this. Its one of the easiest swaps i ever did..thanks for reading..