Hi there- I have a 69 Chevelle SS 396 Clone with a tired 402-- I have a local engine guy building me a custom 427 with roller- dual plane intake- aluminum heads -and long tube ceramic coated. I'm not sure how much power it will have, but I would wager its probably 380+ ( maybe more?). I am currently set up with an m21 4-speed with a 3.23 / 12-bolt ( has been re-geared from a 4.11). I am not sure if I am overthinking it or if I should just drive it til it breaks-- but has anyone done a similar set up to know if the 380 estimate is accurate or if it will be more like 450?-- and to go with that- would I need an M22 to handle that much power ( I would assume I am on the cusp)?. It will never be a drag car while I own it and I want fun streetable power and drive capabilities.



Side note- If anyone has heard of boom tubes-- I picked up a pair of polished stainless steel Speedy's Boom Tubes- will will be straight piped exit out each side- with an electric cut off valve for a true dual exit out the rear to borla atak crate mufflers... can not wait to hear this thing!

Any information or opinions would help!
Thanks

 

For Chevelle questions a better choice of site would be https://www.chevelles.com/

 

A low compression (9.5:1), aluminum headed 427 with a mild hydraulic roller should have no trouble making 500hp.

An M-21 should live just fine on the street as you are typically traction limited.
Mine lived at 632hp in my Camaro, going 10's, but I never launched it over 4000rpm at the track.

That 3.23 is not great with a M-21 2.20 low gear & you would likely be happier with a wide ratio M-20.
I know the 4.10's are bad for highway comfort but they are great with your trans & a BBC that you can rev to 6500.
I ultimately put a TKX 5 speed in mine to keep the performance & get the cruise rpm down.

 

To me, tis torque that kills the beast. That engine sounds like 500 ft/lbs of torque, likely over a wide range, all day long, maybe a higher value than that. In my experience, Muncie cases and shafts are what fail, not so much the gears themselves, gear cut being the main difference between 21 v 22, though obviously GM used stronger gears in the HD trans option M22 for a reason. M21 will likely take it fine if not too many brutal burnouts or drag race starts, if the trans fails, it sucks, but then you’ll know if it can take your combo of power, driveline, traction and impulse control. Or you can be proactive and change it ahead of time, if going traditional, an aftermarket M22W with a “Super Case” might be a good choice, though I’m not sure if they are still available. Or an aftermarket 5 or 6 speed would likely be even nicer with 3.23s due to additional “reach” at each end of the gear spread, with closer ratios in between.

Lord knows, as a spirited ute, with spirited friends, with rather powerful cars, I can recall no less than 4 instances of having to push cars across town to get them into a driveway due to crippled Muncies of all 3 gear set varieties and iirc, it was always the case and shaft giving up. I think at least one we had to remove the driveshaft cuz holding the clutch in wouldn’t even allow the car to roll. We had money to try to go faster but not for tow trucks! To be fair to those poor, departed Muncies, we beat the tar out of those cars and the breakage was iirc typically preceded by horrendous burnouts, donuts and figure 8s, with up to 4 occupants.

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No matter 396, 400, 402, 409Q, 425, 426, 427 454LS, 455W, GS Stage, RA IV, SD, 500 Cad... whatever you like a 427 of any flavor falls right in there with all desirable old school torque monsters.
The M22 will handle 380-500HP, the latter a properly built LS6 spec Muncie. Whats your torque number? That's more important than HP. Torque breaks shizit.

A 68 69 Chevelle is on my bucket list along with a 67-68-69 SS427 ("Imp")...with at Muncie and T83s. Name another Big Block Full sizer that's sexier.....OK a mid-night blue/buck skin, deluxe interior 66 GS MZ code Riviera...But no stick!

A Muncie is old school, the cool cannot be denied, ask the Grump he broke plenty. But plenty of modern 5-6 speeds that will bolt in. You get modern engineering, replacement parts AND vastly increased durability. I repeat bolt in and takes a beating.

 

4 speed guy here. The 3.23 will be the killer. Stick with the M20; the 21/22 are pointless behind a big block.
You're dealing with 50-60 year old metal in the drivetrain. It's not just the case; it's every part in the transmission is susceptible to failure at this point. AutoGear makes new Muncie parts, but the cases are out of production. They offer their own gear spreads that feature the straighter cut helix gears of a 22 with the steep first gear ratios and wider spreads. The 3.23s need as much 1st gear as you can throw at it.

 

I used to be a 4 speed guy. The "wide ratio" M20 is good for 3.42, 3.23, 3.08, etc. The "close ratio" M21 is great with 3.90 or steeper gears. I loved my 400 "E block" with W-30 cam and 4.10 and 4.33 gears and M21.

I raced on the street and later on the drag strip. You were never able to make full use of the engine torque until you had the drive line engaged and could put your accelerator foot to floor. You wanted the engine at peak torque RPM's when the clutch was out. You shifted at or near the engine horsepower peak without dropping below peak torque. Typically, you wanted the engine operating RPM's between peak torque and peak horsepower.
....My two cents worth.