Dean,
A question you need to ask yourself at this point, and you need to be brutally honest with yourself in the answer, is, "why do I need the engine to turn ≥ 8K rpm?" If the need is testosterone based, then go for it if you have the bucks. If the reason is more pragmatic: my ZF has a final ratio of X:1 and I will require ≥ 8K rpm to achieve N mph; then that is cool, but you are limiting yourself to a single variable.
So, really the issue is with the costs of the cars speed over say 165mph. Keep in mind though that there are two variable that can be manipulated to get to the speed range being sought: engine rpm and gearing. Either way getting over 165 mph gets expensive. But, if your engine has the grunt (500-600HP) at a cheaper 7-7.5 K rpm (and will probably have more longevity as well) you may actually be cheaper to look at gearing. Although, I think that this RPM will yield > 200mph with either a ZF 5 DS 25 or a Porsche G50/50 with stock gearing. (By my calculations my R302 iron block based engine with a target redline of 7200rpm, calculated to produce right at 500HP, attached to my G50/50 should have numbers almost identical to the new Ford GT: top end ~212mph.)
With the weight of your alloy engine (that's what I'd have if I had the $$) and the kinds of power you can very safely get out of it at < 8K rpm, acceleration is NOT going to be an issue. But there is another issue that you will need to address if you plan to take the car to speeds over 165-170mph: aerodynamics. The plain GT40 Mk I body shape and some playing around with different front and rear ride heights does good up to ~170mph. It may not take off like the
Mercedes at Nurburgring after that, but it can become a very light, twitchy handful. Those early GT40 drivers at Le Mans and other high speed tracks had to have had a wheelbarrow to carry their testicles around in. Things like chin splitters, adjustable spoilers and diffusers need to be seriously looked at and fitted.
OK, there are a LOT of issues to consider when preparring a car to go VERY fast. Once you've looked at all of them and their costs, one must ask himself, "Realistically, how many opportunities will I have to attain the kinds of speeds this car is going to be capable of?" If you are aware of a racing series whose rules will allow this car to compete and you plan to compete regularly, the answer may be "often enough to make it worthwhile." If you have sponsors who are forking over baskets full of money, the answer may be, "I have to do EVERYTHING I can to win!" For the vast majority of us, the answer is not often enough to justify the costs.
The initial enthusiasm for this hobby/past time/profession, what ever it is for a given individual, makes it VERY easy for ones head to overload ones ass. But, if you ask yourself all of the right questions and the honest answers are the right ones, Katie bar the door and go for it full on, balls to the wall just because you can! We'll all be glad to share the ride, help you when we can and gain some real and vicarious enjoyment along with you!
Regards and Good Luck,
Lynn