I would keep the 250. Everybody (including me) has a SBC. Try a GM HEI and headers first. Tom Langdon at Stoveboltengineco.com will get you a rebuilt factory HEI with new cap and coil for $95. He also has rebuilt Holley Weber Progressive 2-Barrel Carburetors for $80. You will need a $15 adapter to fit the stock intake. Clifford has headers and their 4 barrel intake is better than the Offy. I would suggest a holley 390 and Clifford 270cam if you want to change your intake. Tom Lowe at 12bolt.com has some dyno runs with the 270 cam where he got 270hp from 250 inline.
The heads are the real weakness of the Chevy inline. Bolt-in lump ports really help the flow. Tom has everything even turbo manifolds.
http://www.12bolt.com/home He also has several projects on Inliners International forum.