Exhaust system after the swap
 

Right now I have the 3.4L swap running through the stock 3.4l crossover into the driver's side dump from ORS. From here I have a small piece of pipe from the stock T100 that I had cut and rotated so that the o2 sensor could be plumbed in without hitting the frame rail. From here I had someone weld the stock exhaust system to this pipe. When this was done I realized that the T100 pipe was probably 2.25 or 2.50 inches in diameter the stock pipe was at least .25 an inch in diameter smaller.

What I'm worried about is if the stock 3.0L had trouble breathing through this exhaust pipe, how much more of a difficult time is the 3.4L having. I realize that most of the exhaust restriction is due to the terrible header design on the driver's side, but how much of the restriction is also due to the exhaust system.

What has everyone else done?

I think the main issue on the 3.0 was how it was combined by just looping it and cut a hole on the bottom for exit......

The 3.4 is much better already. I took my truck to the muffler shop and have them put all new stuff in ...I kept the muffler though. I don't know what diameter they use but it meet the CA ref. standard.

When you mention all new stuff what are you referring to? This is going by memory as it's been a while since I've reviewed smog law in california, but I think california does not allow the change on the cat unless it's certified for that vehicle. Because of that I would think the diameter of the cat is still the same. As for cat back exhaust then I remember that you could change the diameter of the exhaust as anything behind the cat as long as it meets the noise guideline should not be illegal.

I'm just wondering if I should consider getting an exhaust done after the swap, and if I should consider getting everything including the cat done, or just run a cat back exhaust. I can't imagine the stock 3.0l exhaust being great for the 3.4l, but at the same time without an aftermarket option I've had bad experiences with custom work in the past.

Do as little as possible for aftermarket stuff. Get it pass the ref. then you can do what you want.
I told my muffler guys to put it whatever it takes to pass the ref. The cat is certified for the T100 engine I put in.

I'm not in California right now so technically the sky's the limit at this point.

I'm just wondering what everyone else did, I'm thinking towards a cat back exhaust but then the former engineering student in me thinks of the neck down from 2.5 to 2.25ish or smaller for the stock piping.

Look through some aftermarket exhaust system for the 3.4, see what diameter pipe they use as reference. I'm sure you can also search the 3rd gen. forum and tacoma forum.

i used obx headers modified to crossover underneath joining the stock location 2.5" downey full exhaust system i had for the stock 4cyl. sounds good to me.
check it out below.

3.4 stock pipe is 2.25" I'm running a sc and 2.5'

Custom built? Where from the stock cross over? or cat-back?

i used a small part of the 3.4 down pipe, cut it just before the O2 sensor and welded that to the stock 3.0 exhaust. works like a charm