do a search there is some good olds 403 info already here. a stock 403 has potential, it has a bigger bore than a BBC . a good set of early 350 olds heads is the way to go (350 olds heads need the head bolt holes enlarged to 1/2"
). The 403 responds well to a bit more camshaft and compression. use earlier 350 heads ('68-'72 will do it, but the '72's are the best) to raise the compression, and an Edelbrock Performer-grind cam (these cams are available from other sources than Edelbrock, who I suspect purchases them from another supplier).
A stock 403 Olds engine has tons of torque. Carbs aren't the problem. The problem is the heads. Their combustion chambers are too large, around 83ccs. This results in a compression ratio around 8.0 to 1. Try to find a set of 350 heads from 1973 or before. This will raise compression enough to even think about HP.
If you add 72 or earlier 350 cylinder heads, your compression will be abound 9.5:1-10:1. It will increase your HP considerably, though you'll probably want to add a cam that can take advantage of that high compression.
forget the 455 head idea unless your pockets are running over with money.
small block olds head info here
some good Build up info here.
a Good site for 403 Olds and all Olds info http://www.realoldspower.com
there a guy on the ROP Olds site in Omaha Ne. goes by Mr. Olds that will talk to you on the phone (super nice guy) and knows his Olds and the 403. Terry FCR PERFORMANCE .com 1 877-215-1800 TOLL FREE
OLDSMOBILE PERFORMANCE PARTS AND ENGINES. We sell Kooks headers Ultra Dyne cams and most other performance parts for Olds.