77 olds 260 5 speed to 403?
 

I'm new to the forum, hi all. I recently purchased a 1977 olds cutlass S with a 260 5 speed (not even on the road yet). The car is in good shape with a recent paint job and mostly original. As you know the 260 motor is a dog but wanted a factory manual transmission. I want to put a 403 in it and have some questions for the Oldsmobile Pros out there.

1) should the 403 drop right in with same motor mounts?
2) if i am not puttig the pedal to the metal all the time will the factory 5 speed hold up? or would i better off finding a saginaw, Muncie or Borg Warner?
3) Lastly, the 403 core i am picking up dirt cheap to rebuild has 455 heads. the motor is seized and wonder if this could be why?

thanks for any opinions of assiatnce you can offer