Tapered seat Denso Platinum electrode spark plugs may not be available for your 350 CI V8 engine. It depends on which SBC heads you are using.
Tapered seat spark platinum spark plugs were introduced for 1971- up SBC heads. They are Denso PTF-16TT (hot) or PTF-20TT (cold).
Standard seat copper electrode spark plugs with a washer are for 1970 and earlier SBC heads. The standard spark plug is Denso W20PR-U (only one heat range).
I use PTF-20TT tapered seat spark plugs in my 1991 4.3L V6 for stop and go city traffic as well as high RPM road driving. I have only changed them once since the engine was built. The were like new (white) at 4,500 miles but i changed to the colder heat range anyway. I checked three of the plugs again about 6,000 miles and they are still prefect. I will change them at 25,000 miles even though they may still be in good condition. In a grocery-getter engine in good condition, they could last for 85,000 miles. I like spark plugs that last for modern crowded engine compartments because it is not fun changing them like it was in the '60's.
Google one of the Denso websites at www.globaldenso.com
and bring up the appropriate chart for the engine that uses your heads. Then click on "details" in the RH column and it will give you the specifications of the spark plug.