Your dropping cylinder pressure as the 69s are 69 CC's !! and the add'l flow WITH the drop in compression will make the engine slow to respond and doggie on the low end!! I put a set of those trick C9s on my 289 in the mid 70s. 1.94/1.60 valves, mild gasket match, changed out a set of ported 289 heads w/stock valves... the loss in compression and low end cost me .3 in the qtr!! I went back to the 54 ccs of the 289s and gaind it back!! They are , however KILLER for forced air!! just a thought!
Just me............................
Thumper