Hi Tony,
The fortunate thing about all the various versions of GT40 reproductions is that they have an ancestral history and the family tree is mostly known. The space frame CAV cars evolved directly from the GTD design. Like Ron says, if you post a question with a picture of your starting point it should be similar to that of other cars with similar recommendations forthcoming.
Although it might not be apparent, all my modifications have been totally focused on making everything on my CAV fit and work 100% as well as on a lowly base model Toyota Corolla. A GT40 that met that standard would be a dream car. I tell everyone to just do little things and make whatever you have feel really good and be fun to drive around on the street.
I really think that a car purchased used is the absolute very best GT40 value in the market, bar none. For 99.9% of car nuts and spectators if it just looks like a GT40, and it is fun to drive on the street, that is all that matters. This is the most bang for the buck.
GTDs have been around for a while and have a good reputation. Likewise CAV space frames are almost the same as GTDs with some features of the CAV mono cars. However, most of the design differences are not apparent and not really important as long as the cars are safe. Regardless, my point is that all the drivability things I am doing to my CAV would also have to be done to almost any particular car anyway. No replica is even close to production car standards in stock form.
A CAV (mono at least) will easily accept a 427 FE engine and ZF trans with only small modifications. However a 351 stroked to 427 will also easily fit, is less costly, and probably is a better engine. I considered both of the above but picked a small block Ford stroked to 342 cu in to save almost 200 lbs and also to have a much lower center of gravity. I intentionally picked an engine that would make peak power around 6,000 rpm and had a very flat wide torque curve from 3,000 to 6,000 for drivability.
My ZF transaxle has a 5th gear ratio so that at 75 mph the engine was only turning 1,900 rpm and the car is very quite at sustained cruising speeds. Whatever transmission and ratios you already have can probably be massaged with a ring and pinion swap to get them into a better street range. Goodyear F1 are excellent for the street because they have the #1 best low road noise rating and the #2 best handling. With the addition of DynaMat insulation in the cabin and engine compartment the sound level inside my car went from 104 to 85 db which is about the same as an older Corvette. The insulation also made the A/C totally adequate even for 120F days in Arizona.
The too forward and cramped driving position in my CAV mono was easy to fix in three steps. What I did may also work for a CAV space frame car.
The seat itself was improved by a series of small adjustments. A drop floor is easy to add as shown in the attached picture. The CAV mono has “tube spacers or standoffs” that support the seat adjustment rails. They can be shortened 1/2” to lower the seat and give the preferred seat back angle. (The adjustment rails can also be eliminated and the seat mounted directly to the floorboard like Howard said.)
Inside the seat bottom, the left and right side 90 degree square edges can be beveled or bent or flattened to 45 degrees which lets the driver sit lower inside the seat bottom frame itself. The seat bottom and seat back padding can easily be replaced with a thinner high density foam to gain another 1/2”. On my car I also raised and widened the sides of the seat about 8 inches. If this was a track only car I would have also raised the edges of the seat for more hip and shoulder support but that makes the car too hard to enter and exit for most people.
I used closed cell foam and did trial and error shaping until it felt right, and then re-upholstered. The tricky part was making it ok for small women and large men. (Or small men and large women not to be sexist.) I went back and forth trying the seat and then shaving the foam thickness a little bit more each time until it felt right for several sizes of test subjects (actually employees who happened to be in the warehouse). The "butt" foam is soft and you sink into the cushion while the side foam does not compress much.
The steer wheel was raised 1 inch by just shorting the bracket under the dash and making a small clearance cutout in the fiberglass under the center mounted tachometer. On mono cars this adjustment lets the driver see more of the gauges through the center of the steering wheel and gets your elbows free and clear of your body.
The stock pedal assembly, which already has fore and aft position holes, was moved an additional 2 inches farther back towards the rear of the car so the driver seat could also be farther back. I am 6’ tall and I keep the seat adjusted back almost touching the engine cover bulkhead.
Overall the seating position feels real secure and DRAMATICALLY change the experience of driving the car. These changes also have other safety advantages. The driver's body is better supported by the sides of the seat. The driver sits farther back in the car away from the dash, windshield, and steering column. The driver's feet are moved farther back behind the line of the front tires. All three changes make for a more comfortable driving position, easier entry and exit, and less venerability in the event of a front end shunt.
Anyway, have fun with your car. If you want more information on any of this stuff, I am glad to help.
