The bigger the mill, the bigger the head the mill can handle. I don't know what that size is on a PM-727, sorry.
If I could only have one head in my hobby shop, it would be a Criterion DBL-202 head. These come in two flavors, a 202A or 202B; the former takes 3/8" bars and the latter, 1/2". It has three bar positions, two on the bottom and one horizontal position. With bars in these positions, you can bore from 0.050" to 6.687" holes. The holes each position can bore to overlaps so the range is seamless.
The leadscrew is calibrated in direct reading 0.001" increments; dial in 0.005" and your bore gets 0.005" bigger. I have found this head to be well-balanced, very accurate and versatile. They have a 7/8-20 threaded back to accept shanks of various types and sizes, including R8.
Note that boring range is sort of misleading. The larger the bore, the greater the centrifugal and centripetal forces are so balance becomes an issue with big holes. If your mill can go slow enough then you can go bigger - go too fast and the mill will vibrate and can even take a walk across the shop.
Criterion no longer exists but you can buy NOS or used heads on ebay. Brand new, these heads went for close to $300.00 retail but new and used heads in good shape can be had for considerably less. If these heads have a weak spot, it would be the way the dovetail was cut. There is a tiny bit of metal at the base of the dovetail and if the user failed to adjust and use the head properly, the dovetail can snap off so look at pics carefully to avoid buying junk. Make sure all the screws are there and ask if the ball bearing under the gib screws are also present.
The DBL series was one of the most copied boring heads to come out of Asia. You can buy an exact copy for about half what the original cost. Personally, I would go for a Criterion myself. I have two heads from this company and they have been trouble-free and precise.
EDIT: I should clarify that one dovetail on a Criterion head is slotted to create an adjustable gib. The amount of metal at the base of this gib is very thin so care must be used when adjusting it. It is more than adequate for normal use but an ignorant user can eventually snap it off.
