Mastkey,
If you want significant improvements on your top end, you need to do one (or a combination) of the following three things.
1. Increase your engine output. Bigger engine or rebuild the old one with higher output items.
2. Install an overdrive transmission.
3. Change the rear end to one of a lower final drive ratio.
Your oil "blow by" could be the rear main seal. If you have a spot leakage between the engine and tranny then it's either the rear main cap seal, the rear oil pan rubber seal or the front tranny seal. If you don't have a spot then it could be a simple oil pan gasket leak.
If you consider replacing the 283 a 350 would drop in place with no mods. So would a 292 inline 6. Your choice.
