Here's my take on the 3.0 to 3.5 swap. The 3.0 & 3.5 blocks are very close. The 3.5 block is slightly wider and has a higher deck height. I know on the Gen II's, a different bracket between the motor mounts and the frame is the only difference between the 3.0 Gen II's and the 3.5's. I also know that the flex plate from a 3.0 will bolt up to the crank and also the transmission will too. At least this is what I've gathered from the 3SI guys who do the 3.0 to 3.5 swap. Here's a link with lot's of information on a 6G72 to 6G74 swap.

I think the easiest would be to use just the 3.5 short block and bolt on the 3.0 heads. They are interchangeable. Also, reuse as much of the stock accessory brackets (keeping the stock locations) as possible. I would look in to using the timing belts and covers from a SOHC 3.5, plus the lower intake plenum. I'm guessing the upper 3.0 plenum will bolt on, but haven't confirmed this. Also, by using the stock heads, you can keep the stock ignition system. Fuel injection might require higher flow injectors, but otherwise I think it would compensate for the greater air flow.

If I was to do this, I'd start with a 3.5 SOHC rebuildable core. Rebuild the short block, then set the two motors up side by side and start mixing & matching parts. I'm sure there are small details that would have to be worked out but bet it's all doable without too much effort. You won't get the same power output as a stock 3.5 but I'm guessing it will be better than either the stock 3.0 or even the 24 valve SOHC 3.0.