New Engine-What size balancer?

[Slonaker]

I am about to order a crate motor from Summit. It says it comes with timing tabs for both sizes of balancer. What should I get?

I'm not sure what size the original 305 has. I'm not re-using the balancer. It has a groove worn in it, and so does the sleeve a previous owner put on it. I tried to measure it last night, but I could not get close enough to it without jacking the truck up, and it was late and COLD.

Will my stock pulley fit regardless of size, or do I need to match what is already on there?

Summit lists a 6 3/4 in their "suggested parts" section, but does not say why.

Thanks,

Slonaker