What you anticipate sounds very much like the modifications I have in mind for my '52 444. However the expenses...
What I've got installed so far is disc brakes, a B20F and an M40 as an intermediate solution, though especially the M40 is far from ideal. I already acquired an AW71 fourspeed automatic transmission and had the bellhousing modified so it can be mated to the B20. However the cutting and welding is something that someone will have to do for me, which will make it an expensive project.
A V6 will give you more torque and more power of course. From the space point of view obviously a small angle V6 will be the easiest to fit. There are ready-to-fit installation kits available for a V6 into an MGB, Triumph or Jaguar for instance but unfortunately not for a PV. Yet it is inspiring to have a look on the sites of the companies that supply those kits:
http://www.classicconversionseng.com/
http://www.johnscars.com/mark.htm

An alternative which you (and me...) most certainly should consider -it has already been mentioned- is keeping the B20 and fit a supercharger. See www.v-performance.com for more details. It would be interesting to hear some reports from someone who has fitted one. On the web you can already find quite a few reports on a similar supercharger for the MGB, that can be bought at Moss Motors: http://www.mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=37878

Let us know which way you decide to go and how you get along.