Likely 2 bolt.
As for the LT4 conversion, I don't see why it wouldn't work, but it's a crap load of $$$ for the conversion parts for very little gain.
Get a set of aftermarket heads (or a good port job on your heads), a good Crane or Comps cam, port your intake and get a bigger throttle body, and get a good dyno tune. Also, remember, the Caprice also came out with the L99 (4.3 liter LT1). They look almost identical, except for the 4.3 stamp on the rear of the block
Tim