how to id big bore kit?

Well, you can pull the head and measure the bore and stroke yourself to see which (if either) is stock.

I think 139 big bore pistons were pretty common, so it would seem odd to stroke it and not bore it too. One guy that offered stroker kits was Rick Peterson at RPM (Rick Peterson Motorsports), but not sure if there was a 139 stroker/stock piston combo. I think RPM got a lot of magazine ink and made a fair amount of sales, but his high prices and some of his claims were a bit questionable, if memory serves me correctly.