Parking is the big issue - you might want to have it more or less figured out before you go and then GPS. The other big issue will be traffic - both in Tacoma and as you come in to Seattle. Probably go after morning rush hour, but you may still have some traffic.
Space needle is at Seattle Center, north of downtown. You might also want to check out Pikes Place Market which is in downtown, on the water front. Unfortunately, parking in downtown is going to be even worse. Have fun - see you tomorrow evening!
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1976 Silver/Firethorn. L48, 4spd. Original 2 bolt, vortec heads, 9.4:1 CR, Speed Pro Cam: 224/224@0.050, 112 LSA, Eagle Steel Crank.