since not turning hard on rpms the valve job is critical for sure. a better valve will help on the low lift flow numbers for sure. your guys should know how to work this to their advantage. OEM will work but wanting to go to the next level of low end torque better parts. My 400 4v headed engine will work in your rpm range but stepped up on quality parts for the occasional haul azz time if needed. Intake pull intake and check and see if intake is machined correctly. clean up intake gskt area and set intake back into engine and look closely at the fit. feeler gages help on doing this check. if the gap is different on front to rear to head and intake machined surface you need to verify these to help troubleshoot you problem. don't assume they are machined properly. sometimes the head dowels are not correct sometime. old school machining back in the day situation. look closely at how everything is setting on block and looks even. gone through this problem before on occasion. just part of the job, assume nothing!!!!