I'd go for a shorter pm nap and regular BT but you might find your LO has another opinion! You may find for instance that he really does not do well, fussing and so on, with a shorter nap and actually does better with the 2 naps of 1.5hrs and a slightly later BT.
If keeping BT the same suits you then try it out for a week or two as you increase the A time, you can always switch later if needed.
This is one of those times when the routine really is dependent on the individual needs. Some like a shorter morning nap, longer afternoon nap, some the other way around and some do better with 2 longer naps and a slightly shorter night for a while before capping one of the naps. I wouldn't cap both as a 1.5hr nap is more restorative than a shorter nap, so keep 1 long nap plus 1 CN.
hth