If all you're doing is replacing the HF driver, it should not change port dimension at all. Cabinet and port dimensions are only relevant for bass alignment. (Although, if you're mounting the HF horn into the cabinet, you might need to increase cabinet dimension slightly to account for the volume displaced by the horn.)

Going with the 8-PI as Wayne suggested is a good option. However, if you're sticking with the standard "bookshelf" format, you will need to introduce a crossover between the Alpha10 and PSD. The crossover designed for the Theater-3 would work well in this case, but verify with Wayne.

Gar.