I had the identical problem with my 76 H16 mast base. I brought it to a local welder who built the area up with aluminum weld and then ground it down to the general shape and drilled the hole through. It isn't rocket science, so just keep grinding or fileing until the step looks right. (remove the tramp before welding) Another option is to purchase a new or used base somewhere. The dolphin striker is often a problem to remove from an older mast base, so maybe someone else can advise with that. Maybe someone has a whole front crossbar that they would sell you?
There is a good article about soft spots and also for installing deck ports on thebeachcats.com website.
TheBeachCats.com
I installed the deck ports in my H16 and was very happy with the results.
Jack