Yes, very bad idea and the sorry ass way out of keeping a Pontiac engine. It's not like they don't make aftermarket Pontiac blocks or heads IF there never existed another factory Pontiac block on the planet.
Why not a Ford, a Mopar Hemi, Caddy, Olds, Buick, or a turbo-charged front wheeled drive engine/subframe fitted to the chassis?
So here is the real reason why many think Chevy is the way to go - aftermarket Pontiac cylinder heads and intakes are capable of making power far exceeding what the OEM blocks can handle or were ever designed for. So to get to the levels that these parts are capable of, you would want to go with an aftermarket block designed to handle, and take advantage of, what these parts can produce in HP/TQ. This cost dollars$$$. Used LS motors are often cheaply sourced through junkyards, just like any throw away Chevy small block.
People will find ways to keep a "body" of their choice driveable no matter what the driveline, but why it always seems to be the Chevy push is beyond me. Eventually you will not see "classic cars" on the road much as interest in them will die off just as the generation who now enjoys them will die off.
How many "original" cars from the 1920's, 1930's, and 1940's do you see at a car show? Very few. Most of them are modified whether it be the driveline, body, or interior. Very few have original engines and for good reason - try finding parts for a 1930 Frankin. BUT, if there was a big following of the Franklin, then aftermarket would produce most all the parts to keep it running into the 21st century to include engines. With our Pontiac's they do produce most all the parts needed to keep it running into the 21st century - to include several aftermarket block choices. So for anyone to blow smoke up your butt and tell you they are doing a Pontiac to Chevy swap so the car will continue to run into the 21st century is full of himself and flat out ignorant to Pontiac engines, or how to build/rebuild them, and only has the coin to buy a cheap junkyard Chevy.
In my opinion, the Oldsmobile is less desireable than the Pontiac and yet why don't we see all the Chevy swaps going into them? Hmmm, like aftermarket Pontiac, there is a new cast iron block which makes it no excuse to slip in a Chevy. I would re-engine my GTO with an Olds engine before stooping so low as to muck it up with a Chevy - https://www.enginelabs.com/engine-te...iller-casting/