The 425 should fit right in where the 307 was. The 425 just has a taller deck height so you might have some interference issues with the valve covers hitting the power brake booster, or the air plenum. Nothing a little modification can't fix. I am not sure how easy it will be to replace the transmission with a TH 400. I don't know how different the TH 200 is from the TH 400. One thing I can tell you is that 425 would easily shatter the TH 200 that is in the car now. 
Since you got an Edelbrock O4B intake you won't be able to run HEI ignition. I suggest you use the old points distributor off of the 425 and convert it with a Pertronix ignition kit. You will find that most people here will suggest building a Qjet carburetor for that engine. They are dependable and very streetable.
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"Some men are Baptists, others Catholics. My father was an OLDSMOBILE man." Ralphie Billingsley A Christmas Story 1983
"We rei-eign, supreme, ooooh god! Burrito supreme, and a chicken supreme, and a CUTLASS SUPREME!" Tenacious D 2007
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