Hi,
Following link does define the interfaces used for clustering with a pictorial diagram too.
https://www.juniper.net/documentation/en_US/junos/topics/topic-map/security-chassis-cluster-verification.html
Further to talk about best way to convert a standalone to a cluster.
Standalone SRX differs from a cluster in following terms:
#1 Interface name: For SRX340 and SRX345 devices, the ge-0/0/1 interface on node 1 changes to ge-5/0/1.
#2 With cluster you would require to configure "reth" interfaces comprising of one or more interfaces from each nodes
#3 group configuration defining the management and system configuration
My best bet to accomplish a Production Standalone to Production Cluster would be as follows:
#1 Prepare new node1 as cluster by first enabling cluster mode and reboot
#2 Prepare the configuration on node1 for a cluster (apply-groups, reth, chassis cluster config) and the remaining rpouting/policies can be right away copied.
#3 Disable the interface on node1 and bring it live in the network.
#4 Cutover the traffic by disabling interfaces on production standalone and enabling interfaces on node1 cluster
#5 upon traffic migration to node1, disconnect standalone from network (leaving management and console connected)
#6 Wipe off the configuration and enable cluster as node0 and reboot
#7 Halt the box, connect control and Fab links leaving revenue cables still disconnected
#8 let the device boot up join cluster and sync configuration from node1 primary.
#9 Once cluster stablizes, connect the revenue cable left disconnected in step 7
Note: Sample configuration is given in the link above for reference.
The procedure seems lengthy but would safegaurd minimum downtime.
Hope this clarifies and helps.