I have a 302 crate motor 1996 with almum heads , told to use autolight 3924 or 3923 thehottest spark but theres diff ranges in heat hottest or coolest spark I was also told if its to hot the end of the plug cn burn off and fall into the motor and blow my engine.Should I just start h the cooler spark to be safe and if it fail the plugs buy the hotter spark I don't wanna spend a fortune on plugs balso don't wanna blow my moter ,I do have msdgnition also.

 

What heads are on the motor?

 

Plugs are CHEAP compared to everything else you're doing.

A racing or performance motor needs cooler, not hotter, plugs. A "hotter" plug has nothing to do with the intensity, voltage or duration of the spark itself. Maybe you know this, but this is a common misconception.

The heat range of the plug is basically at what temperature it will burn off carbon and such and be self-cleaning. Most people look at the porcelain for "color" but heat range selection is based mainly on the ground strap, it should be about at the halfway mark. Too hot a plug can cause pre-ignition or engine knock. Bad. Too cold a plug may foul.

 

they just say SVO and have a num 061B I didn't see under the valve covers so if theres num under them not sure

 

They would be Fords GT40 X, Y, or Z spec heads. These are a big step up from the cast iron GT40 heads that Ford used on a number of OEM vehicles. I used a set of the GT40X heads on my 1993 Cobra R race car. The exact number of the plug I used I can't remember but the plug is a gasket style and not a tapered seat as the cast iron heads use.
The "heat range" of the plug you need really depends on what you plan to do with the engine. An engine used on a race track would use a colder plug then an engine used just on the street. But an engine worked hard such as in a truck may need a plug that's colder. I've found that with todays hotter firing ignitions and good A/F ratio control due to FI you don't usually foul plugs unless something is wrong with the engine.

 

I would go with a 3924 plug