Wow, lots of expensive options being thrown around here, when some simple bolt-ons to a 327 or 350 will do the trick.
I bought a 65 Impala a couple of months ago. It had a 283/2-bbl, 3.36 gears, and a TH350.
I immediately installed a very basic 355ci small block. Specs are;
2-bolt block .030 over, cast crank, stock roods, inexpensive cast flat-top rebuilder pistons
Now here is the key - a set of EQ heads (Engine Quest - essentially a new reproduction of Chevy Vortec cstings)
64cc EQ heads - 10:1 compression
Mild Lunati hydraulic roller cam, duration @ .050 = 219/227
Edelbrock Performer RPM intake, Holley 4bbl, headers, 2000 stall
This engine runs on 93 unleaded, idles so smoothly you would think it has a stock cam, and is a total pleasure to drive on the street with the mild 3.36 gears.
After two trips to the track to sort things out, I have already run a best of 13.93 @ 100mph in the 1/4 mile. When I am done optimizing distributor curving, carb tuning, and transmission shift points, I expect 13.50's or quicker at 102-103mph.
So, after that somewhat long-winded answer, you can go very quick with a 327 or 350 with basic bolt-ons (heads and cam).