I've got a 36" cooktop on top of a 27" oven. The space is actually sized for a 30" oven. You don't need an actual cabinet to house the oven: just a base, and then you mount a fascia panel, to match your cabinets if possible, with a hole cut to the oven requirements.
There are a couple of threads about it on the Appliances forum. - Several people have done it.
Hope that helps

I'm planning to put a 36" cooktop over a 30" oven with the oven and cooktop matching up on one side (oven not centered to the cooktop). In the left-over six inches, I plan on a slide out RAS peg-board storage unit.

Well, that is what I'm planning.

: )
Pepper

Putting a 36" cooktop on top of an 30" oven isn't a big deal if you can get some fillers and panels to match (and support the countertop properly). It's a much bigger deal to rework an existing upper layout to take a 36" cooktop with the proper 42" vent above it. It does get into "gutting" a lot of the cabinets to do this, so at that point, why not look at tweaking the rest to be better, not just the cooktop situation. 20 year old cabinets haven't reached the end of their useful lifespan by any stretch of the imagination, but reworking so many of them can get to be fairly expensive if you can't DIY, and then you wouldn't be that far monetarily from Ikea or American Woodmark or another budget line which you could lay out much more efficiently from the beginning.