Probably they tried to make it faster with Fiberglass front Fenders and Fiberglass front parts.
Best et 9.89
but could not compete with
Sox&Martin short stroke Hemi cars. Low 9s

Me and some other guys have discussed that.
Im not saying that you are wrong.
Some are saying it it was Harrells 70 Camaro and it was damage in a Accident, sitting there and rotten away.
Some are saying it was Grumpys Toy IV that had been in a Accident and was sitting there and rotting away.
And some are saying it was the 69 Camaro that was Orange that was sitting there and was rotting away.
Some are saying that it was Chained to a Tree or a Pole.
Some are saying that it was crushed after Freddy and Eddie Lafayette (I belive that was there name) closed there Shop.

Harrells 70 Camaro was Orange
Grumpys Toy IV was Orange
BM 69 Camaro was ?

Grumpys Toy IV

I've talked to Doug Boyce about Heavy's cars, and he as a fellow racer agrees with me that most people would not add a vinyl top to a car like IV.

I bet Heavy's brother or Carmen Rotunda would remember the history of each car, because I think there were 2-3 first gen Camaros raced with Heavy's name on the car, but maybe some of the cars were sponsored by him but owned by others.

I can also see where some old timers may have been reticent in the past to connect the dots because Heavy may have had assets seized, and well some of those cars maybe changed hands before that happened.

Just from looking around, I come up with this list:

68 Camaro, orange, vinyl top, grump lump, 427 numbers on the scoop
68 Camaro, former IV car (some say it's the orange one in many of Heavy's pics)
Green Camaro, Carmen's car
69 Camaro, White with orange or red stripes and snorkel hood scoop on SSTs
69 Camaro, orange, black vinyl top, Mopar or Grump lump hood scoop,
70 Camaro, tiger stripes, supposedly a former BM car, grump lump style scoop, no vinyl top
70ish Camaro, black vinyl top, #670 on the top

So did IV maybe evolve like this?

Cars with Heavys name or had J&L Racing Enterprice on them was own by Rufus Boyd.
Johnny Boyd made that clear when some guys were saying that their dad (that was Drivers) own some Cars.
68 Camaro orange ex Grumpys
68 Camaro Green Rotondas (Heavys)
69 Camaro (only one) Orange, White
70 Camaro Tiger Stripe ex Dipalma
70 Camaro orange ex Harrell

No pictures of the 69

That's the first pic I've seen of the green Camaro thanks for sharing.

If one of you guys are friends with Heavy's brother ask him if they added a vinyl to the Grump IV car or not.

No need to ask.
Read the text under the picture and look at the picture

Here you have more pictures of the green Camaro