I would keep them stock until you modify further, no need to run colder plugs now

 

What ever you do, do NOT use BOSCH plugs, I can't speak to the L67, but I know in the 3100/3400/3800, BOSCH has had a BAD track record (I know some of it first hand). But, BOSCH wires are fine.

I use AC Delco or NGK plugs (fun fact, some AC Delco factory plugs are simply re-badged NGKs). I can get a set of 6 platinum NGK plugs from RockAuto for $3 a plug after shipping (cheaper then any parts store for ANY platinum plug).
I don't know when, but GM did start using irridium plugs in their cars. I am running platinum NGKs right now in my Grand Am and wife's Impala. Great plug for the money.

Also, just plan to replace your plug wires. The sheethings tend to dry a little and other lays of the wire where down.

Don't forget to use a dab of plug boot grease (even if they come pre-greased, I like to dab them, the grease pre-installed it usually not enough). The grease helps with futre changes and help keep moisture out.

 

IMO, 103s are WAY too cold for his practically stock setup. The 103 is 3 heat ranges colder than stock (605 is 1 step colder, 104 is 2 steps colder, 103 is 3 steps colder). No doubt these engines seem to be somewhat tolerant to running too cold of a plug, but there is just no reason to go that low. 103s are the heat range many of the heavily modded l67s have run for years.

My problem with them is that you are deviating a lot from factory spec on what is nearly a factory stock engine. I get that the stock plugs are not copper, and that GM wants the plug to run on the hot side to keep them clean even if the person drives it super gentle everywhere. But I also don't buy that the GM engineers were 3 heat ranges off from optimal performance when they selected their plugs.

Don't get me wrong, I'm not saying they won't work, but going that extreme certainly isn't helping and just increases the chance for spark plug fouling needlessly.


Also, I would think a bit about going to copper before you do make the switch. Certainly it has advantages on the L67, that has been proven, but the do need to be replaced far more frequently than platinum/iridium plugs. The cost is basically a wash since the copper plugs are so cheap, but there is a labor component and a chance to damage a plug wire each time you change plugs.

 

I would just go stock plugs, while you at it best if you change the plugs as well, they have a spark plug socket out there the size is 5/8 and it has a rubber insert in it so the plug comes out much much easier