Correct, it has been done before. But the price between SBC/LS engines and Magnum engines is such a difference that I believe the Magnum swap would be the most fun and possibly cost effective setup.
Even trying to find a replacement 4.3L CPI engine is challenging, as from what I've read only 25% of S10/Sonoma/Blazer/Jimmy trucks got this engine. Because of this, swapping the harness and wiring for TBI would prove to be costly. If I could find a 350 TBI to drop in it along with a 200-4R/700R4, I would. But seems like they're worth their weight in gold here, and that leaves the Magnum as a super cheap alternative.
Plus, I genuinely love the sound of a Magnum engine lol. Nothing wrong with "reinventing" the wheel when it comes to a project, especially for the wow factor of, "Hey, that's a Dodge engine in that GM!"