318 0r 360

I can vouch so far for the 8 1/4 holding up and I can see while a properly built 904 woun't hold up. There are small block 727's around even cores are sold on ebay "as is". I would check to see Schumaker makes mounts for the conversion from /6 if you can't find a k member. I used them on my /6 to 360 converion in my 73dart, definitely a thumbs up. a torsion bar upgrade and a brake upgrade are definitely recommended. I'd really research the 400 hp goal though. I primarily street driven vehicle would better be served by maximizing torque, which would be a faster car on the street. Most magazine builds nowadays focus more on building broad torque curves at lower rpm's verus mid to high rpm horsepower not used often with street gearing. Either way, build the car, and smile every time you drive it.