When one considers modifications to these old engines it must be remembered that they are a long stroke/small bore design that imposes several restrictions.
Bore:3.4375
Stroke: 4.3125
About the biggest valves you can fit in this size bore is:
Intake; 1.85
Exhaust: 1.50
Those size valves won't allow enough airflow to make any serious power.
The connecting rod would have to be made longer to reduce the piston compression distance and reduce the piston weight. The piston would have to be custom made at about $100 each. The rods would be about twice that amount ($1600).
I'm not sure what you would have to do about the valvetrain but I'm sure you could easily spend $3000 on cylinder head modifications, valves, rockers, camshaft, lifters, springs etc.
You would have another $1500 in basic machine shop labor.
Then you would have to build intake and exhaust systems with carbs, linkage, etc for about another $1000.
If everything went ok you might be able to make about 275-300 HP.
I'm sure by the time your finished you would have $8,000-10,000 in the engine.
Now you can appreciate why the SB chevy is so popular. For less than $2500 you can buy a Chevy crate motor that makes 290 HP.
Ron
