I don't know if there is a direct platinum replacement for your older conventional plugs but I want to point out that platinum plugs can't be cleaned and regapped like a conventional plug.
The platinum is a coating on the electrodes and if you run them through a plug blaster or try cleaning them up with a file you remove the coating and ruin the plug.
For a daily driver the platinum plugs offer the extended service time and make the added cost worth while. For me at least, I still use conventional plugs in the 250 in my 37 Chevy as it is a hobby car that sees about 5000 miles a year and I like to pull the plugs every winter to examine how the engine has been performing. During some tweaking if I decide to change heat ranges then I don't end up with six hardly used $8 a piece spark plugs on the garage shelf.
With the HEI on that engine I anticipate plug replacement about every 25,000 miles, or 5 years unless there is a problem. If I ran the platinums I probably wouldn't live long enough to have to replace them at 5000 miles a year.
Are platinums worth the higher cost?, yes but not for every vehicle in my opinion.
Mike G
