Heatsink mounting issue

I'm currently in the process of building a Pass Aleph 1.2. This is my third major DIY project after the A75 and Balanced Zen Line Stage both of which have served me right for the past two years. Especially the BZLS since it is completely remote controlled and constructed with audiophile materials. Needless to say they both sound wondefully. The temptation of the Aleph 1.2 is too much and despite the fact that i'm currently in no real need for heating equipment (the temp last week here in greece was 28 C) I HAVE to built it. I just finished shopping for the heatsinks and found some very nice pieces of sinks 30 cm x 19 cm with 6 cm fins. I'm usind 8 pieces for each channels (2 pieces in each side of the "cube") that I think should do the work. The sinks are rated at 0.4 C/W for a 15cm piece but I don't have much confidence in that rating (the supplier is a small manufacturing house that is manufactutring sinks for the local Telecommunications Industry) so I choose to have a surplus of I think 40% in my use of the sinks. I 've already drilled the sinks to make the joints needed for the chassis and the next step is to choose the mounting points for the transistor (IRFP 240). The dilemma I'm facing is whether to mount them tightly together (say 2cm distance between them) of spread them evenly across the entire available dimension (30cm). The second option would mean a distance between them about 6cm (and the same from the edges of the sinks). Both measurements reffer to three transistors in each sinks mounted vertically. As I see it in the first case the transistors will have a more uniform temperature (I'm puzzled though about what is going to happen to the middle on being surrounded by all that heat from both sides). In the second case I think the sink will be used more efficiently since the heat sources will be evenly spead along it's area (I'm worried though about what will be the temprature differential between the devices). I also know (or I think I know) that heat travells upwards. Will that mean that the sink will be slightly warmer on the upper side. That means that I have to mount the devices vertically slighty towards the bottom end? or mount them horizontally and risk wasting the sink? (I' donts think that by mounting horizontal along the 19cm dimension I will be able to make the sink work at it's edges). I' ve never worked with such a large sink, the A75 uses the tube like sink with a blower, and I don't know what to expect. I think though that sinks that big cannot disipate the heat quick enough if the heat source is concentrated somewhere on their centre and you end up using a portion of the sink. On the other hand the IRF Hexfets are, as I've seen with the A75, very sensitive to thermal differences and tend to loose their matching if they donot operate a the same teperature.
Any help ould be greately appreciated.
http://www.rtheta.com do such simulations on their heatsinks, and I think it is on-line.

One such simulation that was done for me was on one of their most radical heatsinks which had a 1cm base and a point insertion of heat. The heatsink was about 550mm long and about 25cm high and the heat distributed quite well even though the fabricated fins vere extremely long, tight-spaced, cerrated and "curled" (reads efficient).

So, I suppose you should look at the thickness of the base plate. An alternative is to place a bar on the heatsinks with the aim of distributing heat.

I once saw a large heatsing holding 5 power transformers with a large and thin cooper bar on it's back to help distribiting the heat more efficiently. It happens that aluminium is NOT a great heat conducer (atleast when compared with other metals), but, it can be anodized very easily, so it's todays standart for heatsink building. Perhaps the copper bar idea could help.