Plenty tough for a 'C'. Check as well with the Network 54 FE big-block and 385 series crowd since many run them in race trim and/or on the street. Doug Nash (of drag racing fame of yore) developed this trans design long ago an alternative to the Toploader by adding a 5th gear but designing it around a tough alum. case. Richmond Gear bought out Nash and has further inproved the line by adding a road-race unit and further beefing up the drag race unit as well. 5th is 1-1 ratio and allows greater top-gear hp & torque transmission than underdriven 5th gears found elsewhere....a prime benefit! Rear gears usually recommended at 3.00-3.31 due to ultra-low 1st but it's still a close ratio box that won't drop-off the rpms from 1st-5th. Mounts in any Ford or aftermarket 'bell with stock Toploader pattern. And yes, I've read that Richmond years ago very conservatively rated the 5-speed at 450 ft./lbs. torque load but... many users run well over 600 ft/lbs. in many applications w/o any failures, especially in drag racing.
Unit uses either a Hurst or Long specific traditional side-mount shifter that fits just about any early Ford body with minimal trimming. Have had one for years (street 5 spd.) and love it! Just ask any users.
"RICHMOND 6 SPEED OVERVIEW (Jag's auto commentary)
This 6 speed is the best 6 speed design out there. Since 5th gear is direct and not an overdrive gear you get nice close ratio shifts ( rpm drops less than 25 percent) all the way up 5th. The original concept of the Richmond 6 speed was that it was based on the Richmond 5 Speed ( the old Doug Nash 4+1). That concept was that in the 1960's to early 1970's we didn't mind having 4.11 or 4.56 rear end gears in American muscle cars. We could have a close ratio 4 speed because we could have a 4 speed with a DEAD 1st gear ratio like 2.20 coupled to a rear axle with a 4.88 gear. Since the gas crunch came along we saw conventional autos having the rear axle ratios drop to 3.08. This would not work well with a 2.20 1st gear 4 speed. So they made 4 speeds with 2.88 or 3.42 first gears to make up the difference. The problem was that the 4 speeds being produced became "ultra wide ratio" gearboxes. This wide ratio thing didn't work for the street people since they didn't want 4.11 rear end gears and a close ratio box or 3.08 rear end gears and a wide ratio box. The Doug Nash 4+1 fixed that problem. It gave you the ability to have 3.08 gears in the rear and have a close ratio "5 speed". The 5th gear was direct and first gear ratio was 3.27 to 1. The newer 6 speed adds an overdrive gear to the mix allowing the user to boost axle ratios to 3.90 to 4.11."
Just one caveat: neither the 5 or 6-speed is cheap!