Gary:
What's it worth is always a tough call. Really hard to evaluate from pics.
Original engine adds about $10k in my opnion. The big blocks always bring more.
Searched for some tha actually changed hands. Most of them don't disclose the sales price. If one has sold at one of the big auction houses in the last few year, that date may still be available.
Found this one, doesn't say what it actually sold for, but they were asking $69k and it DOES NOT have the original engine:
San Fran Z10
It would not hurt to upgrade some of the very cheap things. Maybe put on the correct valve covers. And I am a huge fan of the fender pin stripe. It sets the car off well. Z11 & Z10 were the only ones available with both the rally stripes and the fender stripes. If you know someone who can do that job very well without charging a fortune, I would go ahead with that.
What to ask? Personally, I would start at $60k. That is probably on the high side of one that really should get a frame off. You never know. Someone out there may just have to have it, and may not want to consider anything but an original engine car. If after a bit, you get no offers close to that, you can always come down.