could someone help me with spark plugs mercruiser 1998 502 mag ac mr43t is the rapidfire #10 a replacement plug for the mr43t. and or able to run in this motor

You can run a Rapid Fire #1. It is the same heat range as an MR43T, but has a slightly extended tip and is a direct replacement for the R43TS. I'm not sure if the #10 is direct cross with the R43T.
In reality though, you will not gain a thing by running them over the stock MR43T. You could just run an auto motive R43T or R43TS.

Ive heard that spending the outrageous money on plus liks platinum, split fire etc is a waste and actually isnt good. I have the new camaro and the book says do not use anything but a regular copper top. I mean if you have some big custom engine than sure. But if you have the relative basic 454 502 stick with the stock

Ngkbpr6fs

Why not just run the MR43T? Nothing to be gained with fancy spark plugs.

As far as I know, all Mercruisers today that are fuel injected, ship with platinum spark plugs.

http://www.mercurymarine.com/engines.../mpi8-2lho.php

All the way to the 8.2 Mag. I don't know about Mercury Racing, I assume they'd be specced that way or higher for plugs. I also noticed that the carb motors have a 45 gap, the plats are at 60.

Carb engines gap at .035. The first platinum plugs Merc used were in the 496's. Before that they ran the NGK BPR6FS's and the MR43T's.