Ah... it sounds like you're asking for back-of-the-envelope "simple math" pencil musings... let me know if I got that right. Here you go.

TODAY you have an LTV of 25% ($170K remaining on $670K value). For that, you could go just about anywhere and get a 3.75% HELOC - Pentagon Federal Credit Union as an example [I refinanced with them this summer].

If you take the 50% leverage option, and take out a $250K HELOC...

If you treat it like a 20 year loan, you'll pay $1,482/month (using Google's 'Mortgage Calculator') and the payments should end at about the same time as your original loan. That's $2,882/month in payments which mostly come from the $3,400 rent. [let's ignore the other expenses for now: HOA, taxes, etc. we're scribbling...] This could work well.

Put $250K in VTSAX (or another Total Stock Market index fund), and *ASSUME* the proverbial average 10%/year (YMMV - some years will be less, some better). After 19 years of re-investment, you should have $1.5M in that account thanks to our old super-powered friend compound interest.

The other option - sell the house, and net ~$500K, then:

1) $250K invested in a NEW rental unit that cash flows + $250K invested in VTSAX, 10% average. = $1.5M in 19 years + whatever ROI you can get from the new rental unit.

2) $500K invested, 10% average. = $3M after 19 years [NOTE: 100% tied to stock market long term performance].

BUT... with both these options you're STRICTLY CASH - no mortgage payment (or a low mortgage payment if you buy a new unit for more than $250K) AND no HELOC loan payment.