Ugh that hurts just to look at. Same exact thing happened to my 68 coupe after it sat under a tarp for about a year and a half in my driveway. The roof and trunk lid both got those spider web cracks all over, looked like dried mud. Turns out the tarp trapped moisture against the paint and the sun baked it right through. What really got me was the primer spots showing through where the cracks went deep, no amount of buffing was gonna fix that. Ended up having to strip the whole roof down to bare metal and start over, which I was not planning on at all lol. Hope yours isn't as bad as mine was, you might get lucky if it's just the top layer.
Yeah tarp is the worst thing for paint. I went with a breathable car cover after mine got wrecked and no more cracks. Cost like 80 bucks but way cheaper than a respray.