How do I determine the best spark plug for my motor.  

Well you could go colder till it fouls then 2 ranges hot from that, but really what makes you think stock heat range is wrong? what's your combo?  

What do you mean by "go colder till it fouls then 2 ranges hot from that"? I didn't know there was heat ranges on plugs  

Don't be guessing heat ranges by trial and error, Just go to your local auto parts store and purchase a set of platinum plugs for your application.  

I didn't know they had specific heat ranges. I have always bought Bosch platinum 4-way plugs. I cant remember the actual name of them. The ones with 4 prongs. How do you tell what heat range the plug runs at?  

Each application calls for a specific heat range from the factory, eg. the original plug for a 1969 340 would be a champion n9y. The 9 indicates this to be a cold plug and is used in high compression, performance applications.
The 73 340 used a rn12y, hotter plug, the r would stand for resistor plug, to help keep down radio static.
A stock 318 in 73 would use a rn14y, again hotter and lower horse power applications tend to us a higher number plug.

I use a BCH-4036 single tip platinum plug, they have been in my high performance 360 now for about 4 yrs. without any sign of wear. This would be a good plug to start with if you have a 340/360 engine.  

To elaborate further....
Engines don't care what brand of plugs are in them.
Lower compression higher heat range [cleans themselves]
Too high of a heat range with high comp and the insulator will be so hot it can pre maturely set off the gases causing detonation.
Too cold a heat range and it will foul.  
To hot a heat range can 'domino' to over heating.  

thanks for all the input, i guess i should tell you what i have first,maybe have it dyno'd first?, anyway. 318 ,30 over, flat top pistons, mild cam 270,204,.42 lift or something, edelbrock perf intake,600 cfm carb  

Find the equivelant to a champion rn12y in a platinum plug and you will be fine.  

Give us an idea by doing a cranking compression test with carb wide open and all plugs out. If you have 135-150psi then a plug for stock 340 heat range should be fine.
If you have 135psi or lower then 1 step hotter or so than stock would be cool if you want.