Spark Plugs (Again!)

[Kevpix]

I know this topic pops up occasionally, and I have researched the archives but......... what plugs should I use on my 1970 C3 350?

The plugs fitted are NGK R BPR4FS, they have a taper seat as opposed to the normal crush washer. My concern is that I think plugs with tapered seats weren't used until 1971. My C3 is numbers matching so original engine, I presume.

The 1970 Corvette Owners Manual states R44 or R45 for the Base (300HP). Is it referring to AC Delco plugs? There must be more technologically advanced plugs these days that can be used without any ill effects. My main concern is do I have taper seat plugs fitted when the engine isn't designed for taper plugs? Ho hum

 

[Oneball]

Just because the number on the engine block shows as matching numbers doesn’t mean the heads are original. Take a plug out and see if the seat in the head is flat or tapered

 

[Kevpix]

Oneball. Thanks for your comments and photos, very helpful. I would say that my spark plug seat is designed for a gasket and not a tapered seat plug. I can see a little chamfer or countersink but I think this is just good practise and not a tapered seat.
Also the tapered face on the spark plug shows contact only on the area closest to the threads.
Do you agree? If so, what plugs would you recommend?

 

[Oneball]

looks flat to me too, I’ve got tapered seats on mine and they’re really obvious. I tried to take a photo but it doesn’t show up as well as the one I got from the internet.

You can see how little of your wrong plug is actually providing a seal too.

I like the NGK R plugs.

 

[Roscobbc]

This may help you decode the plugs you have - https://www.ngk.com/ngk-spark-plug-numbering-systems -
TBH it does seem to pose more questions perhaps than it answers. My logic is that a non-tapered plug with washer is most likely used on iron heads and alloy heads safely. The tapered thread plug plugs I've seen tend to be the smaller 'Mickey Mouse' sized plugs used on OEM iron headed engines - TBH I wouldn't be wanting to tighten those-up on an alloy head and run the risk of deforming the mating face on the head. The Brodix heads on my BBC engine are 'Mickey Mouse' sized but with washers and not tapered.
For cross references try this - Spark plug cross reference