I was going to change my spark plugs but I am un sure which ones to get. I have a MSD ignition but I am going to change it to a mallory one, so I need to know what plugs to get. Should I be getting a reach of 3/4 or 3/8?
Anyways thanks. O yea I have a 1969 350

 

You don't necessarily need to change plugs just due to a distributor/coil change. You might be able to experiment with wider spark gaps.
As far as dimensions and style of plug, that should go with whatever heads you're using. I defer to others here, but AC Delco R45S's are pretty common applications.

 

Same as above except always use the coolest plug you can before they foul. I'd try a R43S, you might be able to add a bit of advance with a cooler plug.

 

NGK R5 U Groove plugs.