Line for preventer?

[Brion Toss]

Hi again,
You are likely fine; the bungy is just there to dampen things in use, as well as to keep the line snug when stowed. Remember, even a little snubbing action reduces shock loads tremendously. As for line size, by all means use what you have on hand; just know that the breaking strength for that 5/16" Spectra is probably at least 6 tons. Let's just say you have a generous safety factor.
As for the deck pieces, Sta-Set-X (we are having a special on that right now) or VPC would be better choices than Warpspeed, which is a bit too inelastic/spendy for the job.
Fair leads,
Brion Toss