Very Good Question ! I have a one owner LS-5--M-22 1970 El Camino and a 1970 Nomad Wagon,,, Driver with a 468 & a 700R4. I also have a 010 1/2 ton X Cab. Silverado with a 6.2 ( 413 H.P.) & a 6 speed and the 14 bolt rear.
I am a Geeser and I LOVE the look of a big block between the fenders. That being said, I raced a 427 COPO at the track, YES I lost the race with my 1/2 ton 4WD truck ( By a Truck Fender).
When I mash down on my Silverado with the A/C on, It shifts at 5800 RPMs. and runs on 87 Octane.
To unleash the beast in my truck and go fast shut off the " Traction Control " and use the " Tow Haul Mode ". Then it will kick both of my B.B. Chevys AZZ.
Again I say, NOTHING looks better in a 64-72 Chevelle than a big block ! If I ever do an LS Engine it will be in a 4 door built to go and not look pretty. YEP, My LS project would be a 100% beat up 64-72 Chevelle of some sort, With 40-50 years of patina and not pretty BUT have a go fast driveline.
I love driving my 468 Wagon, With a Q-Jet it always does around 20 MPG. I drive it around 3-4000 miles a year. Many choices with Engines. DO what makes you happy. I do believe my 6.2 Silverado 1/2 ton will kick my LS5-M/22s butt any day of the week.
That big " Orange " chunk of iron, Sure looks BEAUTIFUL in any Chevelle.
P.S............. Your Car.....Your Call !