You can purchase the Ultimate upgrade, and just the needed parts from Kevin at Dynakit. In addition to the transformers and chassis, you will need tube sockets, a choke, bias pots, power cord, hardware, etc. A while back I posted on Audio Circle a complete inventory of what's needed from Dynakit. I believe Kevin has the list, but if not, I can provide the information.
If you elect to start with a new chassis, you will need to combine the instructions on building the Dynakit 70 and the Ultimate 70 as the AVA instructions assume you have a built 70. It should be pretty easy, however.
A couple of notes on the Dynakit parts:
The chassis is an exact copy of the Dyna chassis, but in polished stainless steel. The power transformer is beefier than what Dyna originally used, but has the same footprint as the Dyna. The audio transformers are based on the original Dyna design, and are very very nice.