I've got a 73 440 out of a Winnebago Brave (built on the same model year D300 chassis).

The motor had/has Autolite 23's in it.

I went to the auto parts store and bought plugs for a 73 D300 with the 440 but they are different...

The plugs I have now have a slightly longer reach.

What plugs SHOULD I have in there? It runs fine with the longer plugs... should I get more of that plug or run the ones from the auto parts store?

Pic sucks but maybe you can tell what I mean.

 

The reach on the new plug doesn't look that much different and it is not longer than the existing plug so it should not be a problem. That is not the problem that I see.

Is that carbon blow by on the old plug? The original equipment plugs (the new ones) have the metal gasket, as you can see, that seats and seals on a flat surface on the head. The plug you removed is meant to seat to a chamfer in the head that does not exist in the older heads and may not be seating properly. If the new plugs are what the book calls for (champion RJ12YC or autolite 85)I would use them.

 

The old plug looks like that as I painted the motor with them in it; it isn't blow by.

I do see what you mean by the different seats though.

These are the RJ12YC plugs. I'll try putting them in and I'll let you know!