Hi Peter,
I received your email, and have read your post above. As you likely know, the Aceca chassis and front suspension is virtually the same as the early Cobras. As such, with the appropriate springs and shocks, the 289 should not be a problem for your front axle and I presume disc brakes. The Cobra used a T10 gearbox which should fit fine with the right chassis mount, which I am sure Gerry has at Hawk. The original Aceca Diff may be a bit delicate for the 289 torque, so it may be advisable to copy the Cobra setup with a separate Salisbury Diff.
All that said, the 289 is going to generate a lot of heat and radiator and air ducting will need careful thought. I suppose it all depends what you want to do. With the 289 it will certainly be entertaining, but is likely to be viewed as a bit of a Hot Rod. If you went back to the original engine, the car is likely to have more value should you ever wish to sell. I am a great fan of a tuned Bristol 100D2 which sings like a bird at 6k rpm! I don't have the exact weights to hand but I'd guess the AC and Bristol engines are fairly close, followed by the 2.6 which is quite a bit heavier but produces another 50 bhp. I think the 289 is a bit heavier still but not a lot, and the bonus is another 150 to 250 bhp depending on spec. If you want to race, I don't think the 289 was homologated in an Aceca, so the 2.6 is the quickest engine.
So there you have it. Back to original for highest value, Bristol for the sound, 2.6 to win classic car races, 289 to be a hooligan! Good luck.. whatever you choose will be fun. Andy.