I found the 71 Cuda in the used car lot and figured I'd grab it to run the Amateur Muscle Car event.  After having my ass handed to me I took it back to the shop and gave it the close ratio 5 speed tranny, stage 1 weight reduction, and pretty much every HP upgrade I could give it.  I did an oil change, but couldn't afford the rebuild.  Ok, closing in on 600 horsepower.  I thought I had this race in the bag, but much to my surprise, this boat tops out at 118 MPH.  It takes off pretty fast and handles great, but I get passed out by at least 3 of the AI cars on the straight because I can't exceed 118!
 
Do I need to buy a fully customizable transmission and fix the ratios?  I really don't wanna... I've already invested so much into this thing.

Indeed you do. The close-ratio transmissions are worthless if you've been upgrading the engine. Only fully-customizable transmission lets you change the top speed to a more reasonable figure. Alas, that is ONLY number they let you change...so much for "full customization."

Change your gear ratios, go for top speed instead of quick acceleration.  

Like everyone has said, it's likely the close ratio gear set you got.

Gah, another 20k down the crapper on this car :/

For every race so far, I've just taken a competitive car, taken S1 Weight Reduction and a twin-plate clutch. Pick up some of the $500 race accessories too, they make a difference.

My experience: 
>needs classic muscle 
>70' challenger 
>75k creds (so mad bro) 
> Have to put 50k in to it just to make it decent enough at the top end to win 
> Go to sell 
> 22k offer 
> im so mad bro/10

I did the same with my Challenger.
 
I assumed that the close ratio would have a longer 4th and/or 5th gear...nope. I don't know if that is a mistake (seems like it, considering stock is better in this case) or how they wanted it. It makes no sense to even offer that upgrade if that's what it does.

After getting the fully customizable trans, supercharger, and just about every other performance mod for my Challenger it basically destroyed not only everyone in the amateur muscle car race, but (after a good bit of weight reduction, a suspension upgrade, and grippy racing tires) the supercar races as well.
Side note: It's sort of disappointing that the body doesn't rock when I rev the engine...maybe GT6.
 
And yeah, the resale values on these cars are ridiculous. If I sold my Challenger as it is now I probably would just break 30k cr. On a car that cost like 70-something new with more than that in mods. Is it going by average Blue Book values or something?
 
If I could buy a 1970 Challenger with the real-world equivalent of 50k credits worth of mods on a "new" engine with low mileage for $22k, I'd have one. Maybe I'm expecting too much. It isn't "the real selling simulator" after all and they have to make credits at least a LITTLE hard to come by sometimes to keep you racing.