Do a search on the 6G74 or on my posts. I've thought about this swap in my Gen II and FrankR looked in to it, too. The 3.0 to 3.5 swap has been done in the 3000GT, so I'm sure it's possible here. Here's what I've found -

The 3.0 & 3.5 should have the same rear bolt pattern. FrankR tried to measure and thought there were a couple of boltholes different, but they done this swap in the 3000GT.

Both the 6G72 & 6G74 have the same bolt pattern on the crank. A flywheel or flex plate from one will bolt to the other.

The 3.0 heads will bolt on to a 3.5 block. On the 24V SOHC motors, both share the same head.

The 3.0 in the Gen I uses a distributor, while the 3.5 uses a coil pack.

Accessories are in different locations between the two.

The Gen II 3.0 & 3.5 use the same motor mounts but different adapters between the frame & motor mount. I'm sure this is due to the greater width of the 6G74.

I'm sure the drive train will hold up, if you're reasonable with the skinny pedal. Big Blue never had problems with his 4.3 swap and he used the stock smaller rear axle that's behind the 2.6.

Here's what I've always wanted to try but don't have the time or money to do. Take a 3.5 block, swap on the 3.0 12V heads (which includes the alternator mount and distributor), use the 3.5 lower plenum and the 3.0 upper plenum, use the stock 3.0 injection system, use/ modify the AC mount & PS pump mount from the 3.0, and use/modify the front belt covers from a 24V SOHC 3.5. This would give more power than a 3.0 without as many hassles as the 4.3 swap would entail. YMMV <img src="/forums/images/graemlins/smile.gif" alt="" />