We probably didn't spend as much as you think
In Glenns case I guess the turbo and EFI were most expensive. Mine was pretty cheap too apart from the internals and turbo say. The rest is time and effort.
The difficulty with the carb is getting enough fuel in at high boost. It's a noticable restriction above 240hp or so too but it's been proven a couple of times that they will flow quite a bit more if you push. I think Gianni is mostly managing it with nitrous. A lot of it. To do the same you should install a seperate EFI fuel circuit with regulator. You'll need additional fuel injectors to run the carb much past 250hp. The same circuit can supply your nitrous system too. I think thats going to be the best way to go if you want to keep the carb.
Getting the engine to respond to a lot of boost and nitrous is the next challenge. You should look at putting in bigger exhaust valves and improving the exhaust ports. With nitrous you have a lot more exhaust gas to worry about. Leave the inlets alone. The choice of cam is important too. As off the shelf items go the BP270 is what you should be looking at, with a vernier. Some retard on the cam can make some shocking increases in HP at the expense of low RPM response.
Good luck