Unless I am missing something here, the Capri in question is a 1970 GT, he did not state V6 so it is a 1600 (2L was not in Aussie Capri), it will need strut and brake upgrades for a start, if it was a V6 than this would be different as they use heavier duty struts and brakes.

The Zetec would be a great upgrade as I have noted earlier but it will require current ADRs and unless the Capri has V6 struts/brakes etc it is likely an engineer will require those to be fitted too.

I without weighing the alloy headed Zetec, I would be nearly certain a Zetec will weigh more than a 1600 Kent engine, the Kent engine weighs more than 30kgs less than a 2L Pinto engine.

You can buy V8 kits for Capri's that will comply with engineers requirements.
You will probably find out that similar requirements will be required for a 300hp Zetec, and then you might have emission requirements that might be hard to comply with, as a 300hp Zetec is far from stock, and that engines emission standards have to be complied with.Unless you can pay off an engineer this will be a problem, and if the EPA does a random check you better hope it complies, they can't do much with about earlier standards, but they can with current.

No arguing here, just showing different prospectives of doing such a conversion.

As much as I dislike carbies, I could show links to a cheap carby 350-400Hp Windsor build up using early factory early heads, and I hate to say it, but it won't cost much more building this engine, than purchasing the better EFI intakes etc that will only make a start to do this on a EFI 5.0L, and low down torque won't be such a big issue when the car weighs around 1100kgs with the V8 fitted.
http://www.capriv8.de/perana-engl.html

but as with any conversion there are other things involved..
I do conversions all the time at work and there are all sorts of stupid rules
that make no sence at all that you can use to get around things like the EPA. like the last one i did, fitting a 4L toyota V8 to a hilux and the exhaust noise came in at 102.5 db and the car is still leagle, there are loopholes to use and ways around alot of things

XR8 dismantler - isn't your company mentioned in the recent Street Machine? how much for the TS50 engine?

SR20 - i looked at doing the same to my mark1 escort. I looked at all the options and the SR20 made the most power for the least $$. A zetec and turbo setup would cost well in excess of the standard sr20 set up and would be difficult to get bits for - all custom made - there are kits in the UK but they are all top dollar. The sr20 comes standard with 164kw running only 8psi. Add aftermarket ecu, exhuast and more boost and 190kw is easy. Cosworths are great but are far too expensive. expensive to buy and for parts and to modify. A good cossie motor will cost 10k - an sr20 will be about $2.5k for the motor and gearbox. spend the balance on fitting and add ons and it will piss on the cosworth. Machine off the nissan badges and polish the covers and it will take a few people a while to work out what it is. I do not think there would be too much weight difference in the zetec and the sr20 - if anything i think the sr would be lighter.
The sr20 gearbox is alright to so that would save your the $2500 to buy the supra box. Decision is easy - SR20

If you let one of your customers spend $60k on an sr20 without getting 400kw then you are s**t. that is outrageous and proves that you dont know what you are talking about. 600 hp from a cosworth is definately possible - how much boost though! d**k johnsons car had about this much power for qualifying and was running 35psi plus.
If you dont wanna go crazy with cash then buy an sr20. If you want ultimate horespower to win dyno comps and have an unlimited budget then you might consider a cosworth.
THe cheapest option for an easy 200kw is an sr20.
If you want to talk max possible power then forget both the cosworth and sr20 and put in a twin turbo 4.5lt nissan v8 motor, mid mount it, install a 9 inch, full cage, wheelie bars and you have a winner.
Seriously if you have $10k to spend i would go the sr20 for the better and faster option.
If you have an unlimited budget then do not pick either the sr20 or cossie and buy the biggest turbo motor you can get your hands on. THe real question is how much power do you want and how much money do you have.