That amount of oil burning is EXTREME and may not be solely due to oil seals. Black sooty spark plugs are not an indication of oil burning, they are an indication of rich mixture. I expect that your plugs will be very oily. I would also expect them to be fouled to a point that the engine is running very, very bad.

Before replacing the seals you should do a compression check. If compression is low, it will have to come apart anyway, so you can put new guides and seals in. If you just put in new seals without first checking compression, you could easily be wasting time and effort.

Good luck,