I have to tip my hat to frenchyd. He at least gets the juices flowing, good or bad. Me on the other hand, my posts get ignored, or I'm a thread killer.
Now, an observation. SBC motors and the LS variations, appear to have great longevity. The GM E series transmissons, meh. These with a TH 400 seem to be a fair setup, for endurance style racing. Heat is a killer, for sure. A plan to escape heat seems to be a running theme. Picking the right vehicle, ie: weight, cost of parts, ability to get air flow etc. On the other hand, a Crown Vic P71, or Town Car, may fit the bill. Just look at all the Cabs/Limos that were cruising the big cities for years. Durable, different shifts with drivers/driving styles. Able to withstand heat (idling, bumper to bumper traffic etc.) Plus, the P71 has a "track" record already. Correct me if I'm wrong, because I know zero about Lemons, Chump Car and so on.
One more thing, being a purist is admirable, but in my experience, once you decide to make a vehicle race only, that's pretty much it. You're only a virgin, once.