Normally, I am all in favour of modifying cars, but it has to be said that in so doing one destroys any resale value left in the car.
When it's only a few thousand pounds that's usually a price worth paying, but in your case, with a practically new ARP4, I'd say you'd be better off selling it and getting something else.
Like Tim, I am all for some really quite radical modifications. In the 70s I replaced an ageing Vanguard engine with a TR4 , and then again a TR3 with a Daimler V8. However all these swaps were relatively simple, the engines were on carbs and distributor ignition, so there were no ECU considerations.
Like others here I am sure your simplest course is to sell the ARP4 , it should fetch a very good price indeed to someone who missed out on the original allocation of cars. Then buy either a newly specced Roadster with all the bits that you did like about the ARP4, or get a second hand one and modify it to suit. The third option is to get an older Duratec engined +4 (not GDI) and get someone (Williams?) to drop in a 2.5 Duratec.
Again like others here , I feel that a high revving engine with limited low end torque is not the best for trad Morgan motoring unless you live for track days and racing. The 2.5 Duratec package seems a much better bet
