Here's the big problem with putting an in-line 6cyl motor into a 100...... it's too long. That's why the 100-6 was made longer than the 100, and the 100-6 /3000 bonnet is also much longer. The factory tried to put a 6 banger into a 100 and so they made a couple of BN3s. The body was essentially a 100 stretched at a couple of places and used the new 2.6 liter 6 cylinder motor. They had to make the front suspension into a removable sub frame so they could get the engine into the car from below, because it could not be slipped in from the top. How do I know this? I visited the Healey Museum in Holland last month and I saw one of only two BN3s known to exist. The other is in Australia and I believe it's owner is Patrick Quinn.
If I were going to put a V-8 into a Healey, I would make it a Ford V-8 because the distributor is at the front. But, I would never put a V8 into a Healey. No room for the headers and exhaust pipes. Too much heat....too much weight.
In addition to being a racer of a true vintage Healey race car, I am also one of those "nasty boys" that have a 1961 3000 street car with a Nissan 6 cylinder motor and 5 speed out of a 1978 280Z. Fits very nice and looks good. Because this engine/tranny combo is about 200 lbs lighter than the stock setup, it makes the car handle way better. Isn't that what driving a sports car is all about?