You can go to the newer engine, but the newer six block is smaller. I think
www.stoveboltengineco.com sells angine mounts that will work. The newer block was designed with piston spacing identical to a small block v8, pistons from the small block, and valve train from the big block.
If you're going to the newer six, I would recommend either a 250 or a 292. The 250 is a better engine than the 230, and has more power. The 292 is a taller block, and has a lot more power. With this series of engine, avoid the integral head, which has the intake manifold cast in the same piece as the head. Performance with this head is poor. A good book on this engine has been written by Leo Santucci, which is available from
www.inliners.org and most on-line bookstores.
You could also look for another 235 or a 261, which will involve less work.