about the best you can machine is 4mm you wont get a 74mm t/b but a 72mm. the stock srt6 t/b is 74 so why not just use that? they are all over the scrapyards as they were used on slk 32 srt6 c32 e55 pre 21 and clk430
if you must bore it send it to maxbore.com. 125$ with a new butterfly