My 2003 G35C 5AT has pretty much died, low oil with 172K plus miles.
I was curious if a Frontier VQ40DE bottom end could be used with my VQ35DE top end. I don't if motor or transmission mounts would match up. Or even the heads to the block.

Does anyone know how much work would be involved?

 

Yes. You could even swap the cranks. G35 crank in the vq40 would rev really high and strong. vq40 crank in the vq35 block would have a ton of torque.

You would probably need special rods and heads for each setup.