An overwhelming majority of Republicans and Republican-leaning independents are going to be fine voting for the Republican nominee under a whole continent of circumstances, one of which is picking a nominally pro-choice Vice President in 1996.
But another problem here is Powell's reluctance to run against his former commander-in-chief. Not only are their records somewhat linked, Powell is not going to play the whistleblower here. That's just not his MO. So if he gets picked we have a fairly awkward situation for the GOP where the press is praising their VP for rising above the fray by refusing to dunk his old boss, which likely translates to gains for Clinton.
It's unlikely many moderates flip for the lower half of the ticket, and on Buchanan's best day he might be able to pull off 5% of the vote. In electoral college terms this translates as a landslide to Clinton.
Now if Clinton's not running due to maybe a different series of scandal-ridden events, Powell is off the hook and can say whatever he likes. But then under those circumstances the electoral calculus might favor a more base-pleasing Republican ticket.