So on a modern build, myself I substitute RTV for the side seals. I try to inject it inside the groove as much as possible and also pack that area using putty knife/fingers and so on.
I did that on my current 451 and it worked perfectly. I just made sure to get the seal holder and the block operating-room clean before applying it.
RTV cures from exposure to moisture, inserting a wet pipe-cleaner into the wet RTV not only promotes fast cure but also expands the RTV into every corner of the groove.
