A good builder 2.5 should only set you back a couple of hundred bucks. There have been enough TR6's turned into parts cars that drivetrain parts in need of rebuild (other than overdrives) just don't command very high prices.
It really doesn't take much to get to the 150hp mark. The early PI TR6's (that stayed in Europe) were known as 150hp cars. The only differences other than PI (Petrol Injection) was a more radical camshaft, higher compression and a slightly better flowing head.
Your compression ratio should be in the 7.75:1 range. Pretty low right, it was lowered for emissions purposes. Decking the block to zero piston height is your best form of compression increase. Then if you have to have more you can either skim the head or do a little more decking. The pistons on a TR6 engine can stand proud of the block a bit (a very small bit) without danger. Holding off on skimming the head retains more of the combustion chamber for a better overall performance increase. If you target a 9:1 -9.5:1 CR then you can get up to 150hp or better and still be able to use pump gas.
Flowing the head will also help, bigger valves can be tricky but are possible, though they won't be much bigger. Your better off using high quality valves and optimizing the flow characteristics around them.
As for shops. How close are you to Pandora Ohio? In Pandora you will find a guy named Ted Schumacher, check out his website
TSI Imported Automotive . Another more specific TR6 performance vendor is
GoodParts . Though I have never used them another possibility is
PAECO . Some Triumph performance shops in England are
Revington TR and
Racetorations .
As far as modifying your ZS's for increased horsepower, the most you should have to do is change needles. Any other specifics and a great source for ZS or SU carb stuff is
Joe Curto .
As for headers... I'm not sure but the TVR crowd might be able to help. Any TR6 header will fit the engine but I believe there is a frame interference issue when trying to fit most TR6 headers to a TVR, other than the basic mild steel version like this
Moss Header The one in the back (6 cylinder, short primarys) should fit the engine and frame in your TVR. Otherwise, the stock dual downpipe manifold is good up to the 150hp range.
That should get you started. Beware though, it won't be inexpensive. It could easily be double the build cost of a small block Ford. /ubbthreads/images/graemlins/smirk.gif