The facts:
bought $40000 2/1/1977 multifamily residentail rental property.
never used as his home
sold 9/1/14 contract price $450,000 on installment sale
$45,000 down, $405,000 financed through seller with balloon in 2019 of balance. Depreciated S/L total $40000.

Having issues with tax obligation results as being recognized as not on installment.

Need assistance in filling out 4797 form

Please.

Thanks
Alex