There are already guides out for downgrading both NAND & NOR based consoles. Downgrading a phat 3.70 hasn't been tested, and I haven't got a console to test on at the moment. I have managed to downgrade both a 3.61 & 3.66 console using my infectus.
Best programmer is Progskeet - it's the quickest & easiest to solder compared with the infectus.
Tbf I think you're better off reading some of the guides as the information you need is already there & easily accessible. Open up a word document & copy/paste all the info you find. Condense it, reword it etc... you have a revised guide.