noob valve spring question

The OP’s pic shows the heads have rotators on the exhaust, which usually result in substantially shorter installed heights.

The installed height really should be measured, and corrected it necessary, before installing the new springs.

The 740-16 retainer is for 10* locks.
You would need the correct locks to go along with those retainers
If you go that route you should pull an ex rotator off to see how many grooves are in the valves, so you can be sure to order the correct locks.

The 7* 3/8 retainer to fit that spring is a 744-16, which would use the stock locks.

Aftermarket retainers usually alter the installed height, so don’t assume shims won't be necessary.
They easily could be.