403 was built from 77 to 79. 4.351x3.385. Put early heads (#5 6 or 7)on it and a 220ish cam and they will really haul ass.
if it's a 403 and you can check comp,$500 Olds guys are starting to build these up,but run a girdle(windowed mains)
If you are not making huge horses and keep RPM below 5500, a girdle is not needed. True these have windowed mains, but still a pretty strong block as lon as it is not abused. Great big bore with a short stroke. Low compression from factory, but easily compensated for.