You need to simply find the best auto/machine shop in your area that has lots of experience with rebuilding heads. Make that your criteria. Most "good" shops can do porting and polishing. This should give you that something extra your looking for.
Re: 300E/5sp. There are quite a few people on this site who own them. One example is JCE. He's done several postings. Do a search on JCE for more info on 300E/5sps.
Good luck with the valve job!