2003 Yz250F T-4 on 2008 Yz450F?

I have a T-4 from a 2003 Yz250f and I was wondering if there is any harm trying to mount it on my 2008 Yz450f. The stock 08 pipe fits over the header and gets clamped tight. The T-4 fits inside of my 08 header but the fit is VERY tight. There is a slight gap where a tiny bit of exhaust escapes. If I seal the gap and no exhaust can escape is it still a bad idea to keep the pipe on there? Will the smaller diameter pipe do any harm or could this hurt my engine in any way?

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