I'll only address the cylinder head question and leave the rest for someone smarter than I am.
The 205 and the 225 flow basically the same up to .600 lift
http://www.airflowresearch.com/store...dff4e8910302d4
http://www.airflowresearch.com/store...dff4e8910302d4
The 225 is certainly not too much head for the 418...but the choice is dependent on the camshaft used...whether the headers are an issue, and whether you want room to expand later.
Oh...and thank you for going with CHP.