Be careful assuming
This is pure CYA on the part of MT. IIRC the "Twin Navion" of the 60's &70's used a Lycoming O-340. That engine had a constant speed prop. You might research the Type Certificate Data Sheet for that aircraft and find the prop they used. It would be a good place to start after you check ECI for their recommendation of the right prop to use.
That is a good idea but the props on those old twins where probably steel hub Hartzell's, feathering. Probably will not help much with info, but it's a nice approach.
The "old" O-340 was a little different? I know it had lower compression version for third world use and low octane gas. I use to think it was a "de-stroked" O-360, but I am not sure of that?
The new ECI IOX-340 "stroker" is a 320 stroked for 20cc more displacement with higher compression. This would affect the prop somewhat. I am guessing it would be worse than a stock 320 but not worse than a 360 with electronic ignition, but you need to test each variation of course to be sure, even if it's CYA.
What does that mean? It means they are two different engines. I'm not implying one is worse or better for a prop than the other. Bottom line it has to be tested. With out testing, MT probably would put a 200 hr TBO on the "old" O-340-A1A.
(what I found)
Lyc O-340-A1A, bore x stroke, 5.125 x 4.125, 8:50:1 cr, octane 100/100LL, 170 HP
Lyc O-340-B2A, 7.15:1 cr, octane 80, 160 HP
(Note: just for ref. stock 360 has a bore x stroke of 5.125 x 4.375)
ECI IOX-340, bore x stroke 5.125 x 4.125" (0.25" over stock), 9.00:1 cr, 185hp (?)