I did this about 158,000 miles ago. While I was at it I had the eng. balanced. No problems at all. The shop was nervous about boring the B18 block so much but it was fine. Still going strong except the valves would be in a lot better shape with some leaded gas...Oh,well!

I was reading up on a B18 rebuild until a new motor appeared in my life.

Phil Singher has a good write up. http://www.vclassics.com/archive/engine1.htm

Of specific interest to your question is the mention of "voids" in some B18 blocks. The risk is that when you overbore the cylinders you hit a void and your block becomes little more than an excellent boat anchor.

i bought a 75 b20 from my local u pull it junk yard $130.the rebuild cost b20 to b18 are identical. the advantage better head bigger ports and valves more displacement alt boss cast in head and easy conversion to efi with injector mounts machined in head for future project.you car rebuild a b20 with future plans in the intake and exhaust.

Although probably hundreds of these conversions have been done,
there are a certain number of them that are unsuccessful because they end up with cylinder walls that are too thin. The problem is that all of the blocks are not the same due to differences in the castings. I have blocks that illustrate the problem. So the safer route is to use a B20 block.

John
V-performance.com

Well if you are going to bore and engine out to try for horsepower, just get a B20 and bore it out to .030 or .040 over.

If you just want a fresh engine, then just get what ever oversized pistons you need for your b18....