newbie... is there a way the average slug can measure for rim size

[game-over]

i have a 69 camaro and want to order some 17's possibly 18's.
i have an idea what size the rims need to be but i want to be sure.

any particular way i can measure correctly. I would hate to take the word of someone and eat $2500 for rims.

it is stock tubs with DSE 2" drop leaf springs and stock front with suspesion with DSE replacment and 2" drop springs.
rear disc conversion from right stuff which adds 1/4" on each side or something like that .

i want to run 275's..

i have 275/50/15's on it now with 15" x8 rallys 5" bs and nothing rubs..


any suggestions with what i can do to make this a 100% for me...