I am also curious about when/how an attorney/legal involvement comes into play with this.

I think it's ideal for a 1031.

Again, fair play for a major business like this, but it's the old guard who have held up the forums for years, even with their technological ineptitude.

It looks like R-25 Batts would be ideal being 8" thick to fill the space however R-25 is not readily available from what I can tell.

Cash flow is real, but it’s often a long-term play.

., are all ideal in my opinion. 

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Ideally a HELOC is used for less then a  year project where you can refi or sell to pay back the HELOC. 5 years is a long time, things can happen and if you reduce the cashflow on both properties you'll be increasing the risk and having thinner margins.

Look for a local credit union that can offer a cash-out refinance once the work is done, ideally with minimal seasoning.

Role-playing.     . you paid $300k 10 years ago, it’s worth $660k, (so when you sell you net $600k to make math easy.)and your cpa determined that you owned zero land, so it’s all depreciatable. you owe 15% on the $300k gain plus 25% tax on $109k depreciation recapture, so you owe $70k in federal taxes, then California reaches out for another $30k?