1955 Bel-Air tire size question

[1955_Bel-Air]

I have a 1955 Bel-Air and will be doing a mini tub on it and was wondering if anyone knew the widest rim and tire combo I could fit on it. I was hoping for a 20x12 on rear and 18x9 on front. Any help you could provide would be much appreciated. This is a step up from the builds I have done in the past and wanted to get the sizes correct the first time b/c they will likely have to be ordered. Thank you for your time-take care