Principal payments on a loan do not have any effect on basis. Money borrowed is not taxable and does not increase or decrease basis, loans repaid are not deductible and do not income or decrease basis. For example, when you purchased the home for $50,000 your basis was $50,000 whether you borrowed all or none of the purchase price.
Based on your figures, you would have an approximate $350,000 gain. If you lived in the house for two of the five years preceding the sale none of the gain would be taxable (so additional basis would have no effect on your taxes.)
