My 88 IROC got a bad valve guide seal too. It happened as I was driving along a windy road. All of a sudden - POOF - Huge cloud of smoke in the rearview, for about a mile. Then it stopped, never to smoke again. But a hi-po chip would never work right, it always pinged, I think it had too much carbon deposit in one cylinder. And when the dealer fixed a leaky manifold gasket, under the extended warranty, they said the valve guide seal was bad, and fixed that then too. Anyway, a puff of smoke on startup is almost always bad valve guides, or valve guide seals.
now: '73 LT1; '79 Z28
then: '67 coupe; '88 IROC; '95 Z28