I went with 4" lane spacing on my track thinking it would be plenty, but I wish I had gone 4-1/4" or 4-1/2". I have found that I really prefer the 1/24 scale cars and they race well on the 4", but could use some more room. You really have to watch out going into a turn if you are on the out side. If you are side by side and stay side by side it's all good. If the guy on the inside gets a better run through the corner than you may get taken out. I'm not saying you should go with more lane spacing. I'm just letting you know what to expect.
Harry's water damaged track was 2-3/4" I'm pretty sure and he has decided to go with 4" for the new track. Will he like it better in the end? Hard to say, but I hope to find out for myself.
Keep us posted and enjoy the building of your new track. I'm sure it is something you will be proud of when it is done.