...After Installing a set of Closed Chambers (over the 'Open') You will be Amazed at the Improved performance! At a 63cc 'Combustion Chamber'. I believe, correct me if I am wrong...the Open chambers are 111cc? 117cc? chamber.
At any rate, you are Increasing the compression ratio from 8.0 to 9.5 IF You keep the 'Dished Pistons' (or have they already been replaced?) as Closed chambers with FLAT Top pistons, with valve reliefs give 10.5:1 CR.
Be sure to have the heads Machined for 7/16" Screw-In studs. ala 'Boss 351'. And be sure to have ALL Sharp Edges Polished Off.
The Two 'Major' Parameters that Drastically Increase Horsepower, in My Book, are,
Compression! And Valve 'Curtain' Size, along with Valve Bowl Polishing, the way I went. The Valve 'Curtain' is The Intake Valve Diameter X Actual Vale Lift Off the Seat. And remember, the 'Actual' Compression is determined by the Opening and Closing, Timing and the Durations of Both Valves...Relative to the Position of the Piston during it's Stroke. 2.190" Intakes are Huge on the Cleveland 4V Heads. Some have gone to Aluminum Heads with smaller Valves believing this increases Flow, as It does increase Air Flow Speed, (I) Don't know for sure, may be a 'Trade-Off'. On the Dyno...it's All About 'Volumetric Efficiency'!
Do it! When You're seeking Much Higher Performance. You will Not Regret it!
Just do it right and Complete it. Every piece must compliment the others, starting with the CAM, Heads, Manifold, 'Carb', Headers. And Yes, the Ignition and it's Timing is Crucial, and a Whole 'Nother 'Ball-Game'.
Good-Luck with it!
You don't have to 'Hope' for it!! You WILL be Gloriously Rewarded! HOPE is Not a Strategy!
MJ