I have some extra .458 300gr. JSP. Is it prudent to use these with .451 sabots in a .50 cal t/c? If not does anyone make .458 sabots for .50 cal

You'll need the orange colored sabots from MMP. These are thinner and are designed to be used with .458 bullets.
I'd imagine you would have a pretty rough time ramming those bullets down the barrel with the other sabots, and pressures would likely be sky high. I wouldn't do it!