Basic "BOOT" question about 1031 exchanges and seller financing.

[Justin Summers]

I have a rental property I've owned for 10 years that my basis is about $150,000.  I sold the property for $600,000 and am holding $150,000 in seller financing for 1 year.    I have a property to purchase for $300,000.    I plan put $150,000 cash with with the QI to cover the $150,000 seller held financing. I know about recaptured Depreciation and other selling costs affecting basis and deductions but I'm just trying to figure out the larger costs/taxes.

My questions. 

1.   If I didn't find another property to purchase other than the $300,000 one.. I would owe normal long term capital gains on roughly how much?   

$600,000-  Basis of $150,000 = $450,000 

So If I only buy the $300,000 would only owe capital gains on $150,000? Even though I only reinvested $300,000 of the $600,000 sales price. 

Or would I owe capital gains on 50% of the gain ($425,000) since I only invest 1/2 of the $600k sales price? 

Thanks

Also if I knew I was only going to buy the $300,000 replacement property how would it affect my Capital Gains taxes if I didn't pay the QI the $150,000 to cover the seller financing?  Would my capital gains be the same?