Clydesdales are sweet looking horses... bigger dubs would be needed though...
20" wheels doesn't need extensive work for anything if unless you want to go superwide tires. I had a '05 Mustang wheels in size of 20x10 and they had 295/30-20 tires Toyo Proxess 4, it was before I had the fenders rolled and before the shortened rear axle. Wheels had a enormous bs of app. 7.2", they did not hit the leaf spring or the inner wheel wells. But they were tucked in, did not looked good, would have needed app. 1" spacer. In my experience I could have mounted a 315/35-20 tires to rear with correctly positioned wheel. I rolled the fender lips just in case.
What comes for the tire overall diameter, I have no desire to under 27", looks just too small IMHO. I would use 30" tires rear if they would fit. Well, actually they do fit, but they will rubb the wheel well from the frontsection if you want to drive the thing and IMO its the main purpose of the vehicle. I am using 29.3 tall drag racing slicks which has 9" of thread and 11.4" section widht - they fitt good, but pretty much maximum overall diameter with stock fenders.
What comes for the wheel package, the more you guys are saying "no no" the more I am thinking of "go go". I do have an option to go with 18x9.5-18x10 wheels with 295/45-18, that equals 28.4" overall diameter. Once I got my 17" steel wheels done, I'll share a picture where my Mach is having 17x7 with 27.1" at front and 17x9.5 with 29" at rear. I've shared a quick peak from my garage earlyer but here it goes again. It's been 1 1/2 year from the pictures taken and absolutely nothing has happened for the wheels since exept they have rusted a little
http://www.7173mustangs.com/thread-17-steelwheels
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edit: corrected the rear steel wheels overalldiameter from 9" to 29"