What options "cannot" be retrofit - or at least not with ease

[twobjshelbys]

Simple. If it attaches to the frame and penetrates the skin, better get it done at time of manufacture (e.g., passenger roll bar). If it attaches to the "floating" part of the structure (engine, suspension) go for it.

[1795]

The heater is not that hard to install by yourself, with a friend or two, after you start building the car. It does help to have the heater holes pre-cut. What becomes more difficult is to retrofit a heater after the car has been completed. It can be done, see my thread and pictures, but it is not easy. I did it in a weekend.

[DanEC]

If using a 289 motor you probably don't need the higher capacity cooling fan. ERA provides a good sized radiator and the standard fan would probably work fine unless you have some really hot weather.

Twoshelbys brings up a good example about the roll bar. if my car is typical, it included the mounts for the driver side roll bar, although I ordered a street roadster without the rollbar. If you may want a passenger side bar then that needs to be fabricated in at their shop.