64 chevrolet chevelle looking for bigger wheel

[NitrousRuss]

Hi all can anyone help, looking to get some bigger wheels and tyres for my car. I've been on some American forums but not much help, I'm either looking for 18s front and 20s rear if possible or 18s all round, looking to go wide as possible on rear, I'm just confused with the way they talk about back spacing, so if you can make it clear, and the sizes I could go
Many thanks
Russell