Can an Allison 545 fit on a 350 Chevy?

No, an Allison 545 transmission will not fit on a 350 Chevy small-block engine without significant and likely impractical modifications. The Allison 545 is a heavy-duty, electronically controlled automatic transmission designed for much larger engines and vehicles, typically found in medium- and heavy-duty trucks. A 350 Chevy is a relatively small engine. The bolt patterns, bellhousing dimensions, and overall size differences are insurmountable without custom fabrication that would be incredibly complex and expensive.