As new genuine Honda exhausts are no longer available, where is the £393.63 exhaust from? Is it complete with all six gaskets and four clamps?
MoT exemption is automatic at 40 years, so there's nothing to 'apply for', but until it is registered as Historic (from 1st April in its 41st year, so yours qualifies now, as you know) you can only tax it at a PO, with the V112 (which is a pointless form as they only throw it away or give it back to you - they can see when it was first registered on the V5C you give them!), not tax online. Once registered as Historic, you still have to 'tax' it every year, or SORN it, as before.