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Why ?
Like, if its just a round town car then the four speed will be good enough and a fifth gear would just be weight.
The only reason i have a five speed in my chariot is that i have the low gearing (or is that tall gearing) so that i'm able to tool around the streets in fifth and not have it bitch and complain when i give it a bit.
Most of the parts for the '30 will fit the '55. Most of the parts that are different is cosmetic (eg: headlights, grill, tail-lights etc). Have a bit of a search around the forum its all been covered before.
Getting the gearbox from the celica will need a "K-T" bellhousing will be hard to say the least, unless your also swapping the motor out for either a T-series or a A-series motor. Then it'll be just making sure you also get the gearbox with the (or for that) motor.