Apart from valve material, and I guess valve seat width are there many other things to look out for. I can't see why you would have to run larger valve guide clearance but could be wrong!!

How you you prep a street head for marine use?

The heads are Flotek I've used on street engines before and been happy with them, sure they are not high end name brand heads but the casting are good and they use good components. Part of my business is a small but skilled engineering facility. We have no problem doing the machining work but Like you have pointed out we do lack knowledge between street and marine.
The guides are Manganese Bronze and the seat are also Powder Metal which gives me good bones to start with.

I've just purchased a Rottler 3 angle cutting tool I'm not sure what valve seat width we will have until I cut a test piece, but I think I'm going to shoot for 5% of the valve diameter. We do have engine builders here in Nova Scotia but they are more reman stock engines, plus I like to keep as much work inhouse as I can and learn from the process. Thanks for all the pointers to date

Fully understand why you want the above in a big hp endurance marine build. Where is the line that it becomes mandatory? Look how many hours on stock 454/502 mpi and 496 setups. These are stock GM heads, same as in auto use.