I have zero knowledge of the 270 Ackley. What I do bring to the table is personal experience with the 30-06 Ackley. Same concept! From what I gleam, you are keeping the same barrel and just having the chamber opened, so velocity increase from that end will be zip, nada, none. I went from a 22" factory Rem barrel to a 26" SS custom Lilja, along with pushing my pressures, I recongnized a 300 fps increase w/ 180's (2750 to 3050 fps).
Like you, I wanted to be able to maintain as much of my previous assortment of components and reloading equipment with out having to re-adjust for an entirely different cartridge/caliber combo. I was able to keep the reloading die set up inexpensive by using the wonderful Lee Collet Neck Die. Works great! Though after 3-4 firings, chambering a round becomes difficult due to not setting back the shoulder. Thus my purchase of a Redding custom body die; probably about $70 for you. Call Redding direct for that purchase. I already owned the wondeful Forster Ultra Seater die in 30-06. Forster sent me an Ackley floating chamber which kept me from having to purchase an entire seating die; about $45.
I did some looking around for propellant charges and ran across a guy who goes by the name of "Ackley Improved User" over on www.accuratereloading.com.
He had used an interior ballistic load program (having a senior brain fart at the moment and can't remember the programs name) which when you plug in all the gun/caliber/powder/bullet specifics, gives suggested load data. He then personally tested the load data and discovered a load which was A-typical for the '06 Ackley but worked wonderfully. I too, tried his load of heavily compressed charges of Reloader 25 pushing the Nosler 180 BT or AB and immediately hit upon 3/4 MOA loads at 3050 fps.
Good luck in you endeavor.