I would not take the head to just any GOOD machine shop. Find one who has done a number of these and are familiar with the breed. If you can't find one, I know a very reliable and THOROUGH, small one man shop in Dallas you could ship it to. He's not fast, and he's not cheap, but he is honest, reliable and GOOD.
You may already know all this, but the 300E with 5 speed was brought to the US only in '86, '87 and '88. The best I have been able to determine, I expect that there are less than 500 in the US and probably less than 300. A team of wild horses couldn't drag mine away from me.
Have a great day,