Okay, here's what I love and hate about my Lemans.
Like:
-The simplicity of the driving experience. This is extremely subjective and not for everyone. It's not super complex, it definitely is not smooth or quiet. It has personality.
-It's not perfect. I actually like cars with some quirks.
-When I'm wrenching on it because I want to. They are a lot simpler than a new car to wrench on overall IMO.
-The stuff I fabricate and/or bolt on is mine, I consciously chose those parts based on what's there.
-Surprisingly mine cools well as long as it's moving due to the corner window layout even without A/C
dislike:
-When I have to wrench on something that's broken. I'm also doing what I do on a very tight budget so that factors in.
-It's not as reliable as say the vette you're going to replace it with. Partly factored with above, partly factored with the car being 45 years old.
-Leaks fluids. Sounds stupid but newer engines even say a Vortec 350 doesn't leak oil like a traditional Pontiac.
-Power nothing. This is something that I've progressively lost the romance with. I am going to eventually retrofit power steering and power brakes.
-The good stuff back then sucks today. The rotors that came as the best option on a 71 Lemans were 10" tops, and mine had drums. Mundane 98-02 F-body stuff is 12" rotors even on the 3800 cars.
My advice is
A)make sure you understand the implications of the chassis choice you make in terms of braking, suspension and engine choices.
B)pick something with an aftermarket if possible, mine's a 4 door but it's an A-body. If I was willing to drop coin parts grow on trees for them and you can get nearly anything as a bolt in. Understand the aftermarket before buying
C)Don't sneer at the idea of a "full power" car. I did, lesson learned.
D)Be ready for surprises. This is less relevant if you're going to do a repower, but I had to shove a cam in my Lemans after I found out a lobe had been totally starved from top fed oil due to carbon build up. Totally unexpected though.
E)Don't underestimate other chassis just because they don't look performance. My Astro van has a high degree of interchange with a second gen Camaro in terms of front suspension, as does a C1500 or a Caprice. The powertrain is also largely similar to say a Third Gen Camaro.
F)Perhaps look for a mostly complete Pro Touring setup someone is getting out of. It sounds like that's the way you want to go anyways, and you may be able to get what you want for not a ton more than the car. As said they are not exactly sound from a fiscal investment standpoint.