A/C bracket bolt question

[69Tom]

Have a question for you guys. All the bracketry was removed from my engine for the stock A/C system prior to my ownership. Stock 74 454 truck. Since sourced said brackets, now I need bolts to connect them.

Are the threads in the heads and block to mount the brackets fine or course threads? They seem to be coarse, but I didn't want to screw up the threads by tightening in my test bolts (I still need to get the correct lengths)

Also, should I go with grade 8 for strength or 5 so there's some slight flex so they don't just shear off due to potential vibration?

Thanks.