It looks like the primaries on the Doug thorley tri-y's are 1 5/8". The only question is how do they fit on the car? I bought Hooker super comp's and both my stock steering and my rack run into the front cylinder tubes (on both sides). I do have after market motor mounts, but I don't think that they lower the motor all that much. If these tri-y's wouldn't hit the stock steering components, then they would easily clear the rack. Maybe a little better ground clearance as well? Any experience with these would be appreciated.
With well over 400 on tap, (imho) you should run 1.75 headtubes, unless you are in dire need of more low end torque... then try a 1.625
Also remember if you have them coated (or buy them that way) you will want to run them one size bigger. (ie: 1.75 coated is roughly = to 1.625 uncoated) Not in terms of physical dimention, but in terms of function... the ceramic coating holds more heat in the charge, makes them act smaller than they are.
DD
(f.w.i.w. I run 1.75 coated on my 306 and it likes them just fine. The ones on my 351 are 1.75 uncoated)