My spitfire also has a lowered floor to accomodate my height. The alternative I've seen is to fit a roll bar/cage that is higher than normal above your head. At 6' 3" you'll probably need to do both however.
As AndyV says, come along to one of the TRR/TSSC races and have a look what other people are doing. It's a common problem as the spitfire isn't that high to begin with. I'd expect that anyone taller than about 5' 8" -ish is likely to have problems without modifying something.
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