[nytrainer]

As a comparison I have a stock ‘68 350HO 4 speed w/3:36 saf-T-trac in my Lemans convertible and I don’t think the HO engine is really anything more than what your rebuilt HP/TQ numbers should be although the 18 heads do yield a higher compression.
What I can tell you is that it gets up and goes (factory rating 320HP) so something is off with your tuning I would suspect. I thought my 350 HO came factory as a 068 cam with the 4 speed (evidently ‘69 HO Lemans did) but I was told it should be an 067 (you can hear it idle in my signature). While a Summit 2800 might be a good choice I would have at least chosen the 2801 (with my drivetrain combination).
Maybe have one of our resident experts go through your distributor curve and carb after you resolve bad lifter issues (as mentioned possibly dropping those cylinder down on power)? If that doesn’t liven it up maybe the cam timing is off?
Let us know what you find.