75 plymouth roadrunner 400 to 440 swap

400 is an externally balanced motor, so flex plate will likely need to change. If I recall, the 400 has a short stroke, low deck but shares same block as 440. The 440 has a longer stroke. So most of intake/valvetrain/distributor can't interchange, but everything should. (it should also bolt right into same motor mounts.) You should also be able to keep same tranny, although swapping to a 727 might be worth your time if you have a stock 904. I would verify whether the motors are internally/externally balanced and make sure you have the right flexplate and/or torque converter.

I would keep the car original and rebuild the stock engine. Hop it up if you want to. Your car will be more valuable with the original engine.

i mean the car belongs to one of my friends in the army and back in high school he had it and i would use the same block but lets just say it has a giant hole in cylinder number 5 and i been looking for a 400 block but it was like looking for a needle in a hay stack

Well that is different. Like Ken said the 400 was a B engine and the 440 was a RB (raised block) engine. A 383 B engine block is also an option. There is a 400 block for sale on Ebay for $450.

Here is a source for blocks ---- http://www.indyheads.com/blocks.html

yeah i saw that it was for pick up in california & im in texas haha so i think that shipping would be alot

That car is worth doing right. Good luck!!

One last comment. Try to buy a block that is correct for 1975. There should be casting numbers that can be used to date the block.