You don't want to keep the original bore. It's a lot of work/time/money to remove/replace an engine. If you're going to go through all of that you want to do a rebuild. Fresh everything. The bad part is when you get the engine out, the engine compartment is going to look pretty bad. When these cars were 5 years old there was rust under the battery tray, etc. You're going to start thinking, well if I'm ever going to paint the engine compartment, now's the time to do it. So my advice is before you get started on this project think it over good. Either do it right or continue to drive/enjoy it as you have for the last 30 years.