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I suspect that you have NPT or NPTF plugs (“F” - is for fuel, and uses a slightly different thread form, you probably cannot discern this by eye). The straight thread plugs are unusual. Doug at precision Pumps knows his stuff, and I would call him and confirm the correct tap.
If I had to guess, I would say use an NPT tap and fit each plug to the tapped hole for the desired depth.
A word of caution here – A tapered plug can exert a huge amount of force on the OD of the tapped hole and I have seen more than one casting split by an over-exuberant mechanic, often at great expense to repair or replace.

Often in production machine work an internal straight thread is used with a tapered dryseal plug because it is problematic to control the exact depth of an internal tapered cut thread. Just a few thousandths in dia will vary the depth of the fit by a relatively huge amount. Anyone that has machined mating precision tapered fits will confirm this.

To further complicate matters an internal straight thread tap can be one of several types… see below
(This more info than you need! – LOL)

NPSC – general purpose (pipe couplings)
NPSF – Dryseal American Standard Fuel Straight Internal
NPSI - Dryseal American Intermediate Straight
NPSM – American Standard Straight Pipe threads for free- fitting mechanical joints
(For example, hydraulic fittings often use this thread form; these may use a gasket seal)

There are probably more than twenty various pipe thread forms that you can find today - including British and Metric. With many products coming from overseas this is often an obstacle for the mechanic that deals with Hydraulic systems.