Spark plug indexing washers for sbc 350

I need some indexing washers to index the spark plugs on my sbc 350 but I don't know if I need tapered washers or flat ones. I am using the stock iron heads. It's from a 1970 Corvette.

Here's one of the plug holes:


and these are the plugs:


Any advice appreciated.

 

Tapered plugs = tapered washers;



But I believe 70 Vettes were not tapered plugs, can't tell from your pic.

R43S is the standard plug, not a tapered seat plug.

So I'd worry about which heads I had first.

 

Tapered...I picked some up at the local parts store...

I type slow....hoop98 makes a valid point....1970 would have been standard flat seat not tapered...1971 would be tapered...

why do you want to index them ?

 

Indexing may get you a thousenth of a second on the qrtr mile.... I am also curious...

 

Thank you for your answer.

I heard that by indexing the plugs you get minor improvements in fuel efficiency, a slightly better idle and a tiny boost in power. I decided to see if I could index my plugs without using washers and I got 5 of the 8 correctly positioned without them. So I figured, for an hours work and the cost of a packet of copper washers, I might as well finish the job. It's not for racing, I'm just interested in improving the running of my car.

 

So the question is, what year heads do you have?

 

1971, 76 cc heads.

 

okie dokie, 71 = tapered

 

My driver always seems to run better after I wash and wax it

 

Minor....slightly....tiny.
Rather spend the hour driving.
I suppose the little things do add up over the course of a year.