I'd call Larry at Wells Speed in Weatherford TX. Theres also 'Kajun' Jon Bergeron in Whitney, TX.
I can send you a measured drawing of both versions of the aftermarket 'Muncie' (10/27 spline and 26/32 spline)
If you compare a factory Muncie to an aftermarket version, the aftermarket version is .147" of an inch longer due to the thicker front wall of the Supercase. This will push the trans back 147-thousandths on the crossmember. Your driveshaft slipyoke will normally handle this in a factory installation. I suspect you don't have a D/S so its not an issue. Comparing the 27 spline to the 32 spline, the 32 spline unit (factory or aftermarket) is .700 longer than its 27 spline counterpart. This is where people get screwed when they switch from 27 to 32 splines. It moves the shifter (possibly back or possibly forward .700" ; it also probably means your D/S needs to be shortened and the yoke changed AND rebalanced. If you go from a 27 spline factory to a 32 spline aftermarket; then you have to remember that extra .147" from the Supercase.
With regards to the Shifter, Id go 27 Spline mainshaft, this will give you 2 mounting patterns for the shifter block which will be about 700-thou ahead and 700-thou behind the 32 spline mounting location. I bet a 55-chevy benchseat "Bullnose" type handle would help; or buy the shifter and make your own handle out of stainless or aluminum barstock. Older Hurst shifters and linkages are OK; I've had some reports of the newer offshore made ones having linkage rods that are bent wrong and soft LOL
Buy and old one and then find the right handle, buy the handle new
A GM Bellhousing will work; you should still use a dial indicator to make sure the trans is mounted parallel to the block face and that the crank and input are inline. For a performance application; .005" TIR or less is preferred. For a street car, you can bump that up to .009 TIR.
If you want a scattershield, I really like the Quicktime. But its twice the price of the Lakewood stuff; twice the quality and probably half the weight. If you need dowel pins for adjusting the BH; look into a company called "Robb MC"
Gears: Lets get historical for a moment. The Muncie was introduced in 1963, but that was a far cry from what everyone is used to, its more Muncie 3 speed than anything else. so 1964, the Muncie comes out. The original design was the 'M21' close ratio (2.20 1st/ 1.64 2nd/ 1.27 3rd and 1:1 4th) and was designed for ROAD RACING not DRAG. This had a 7/8th" dia. countershaft 'pin'. Then the M20 was up; similar trans with a wider ratio spread using a 2.5 1st gear. The M20/21 use the same mainshaft gears, just a different input gear and cluster gear. Around 1965; the M22 starts showing up. This is a Heavy Duty version of the M21. M22s have identical ratios, but use a different pitch to the teeth than the 20/21. By todays standards, the M20 would be a close ratio; so 'close' v. 'wide' is usually a moot point in most applications.
Around 1966 the geartrain was upgraded with a 1" countershaft and synchro rings that have a support shoulder in them. This applied to ALL three ratios.
1970 we see the use of the 32 spline (Turbo 400) output and the stronger 26 spline input gear. This was ACROSS THE BOARD; *NOT* M22-specific.
The high quality gears are M22 design. The Italian ones are made of the European equivalent of 8620 steel. The firm that makes the Italian gears is Masiero, their parent company is Euroricambi. They also make the gun-drilled mainshafts (from the front, in the green state, and then hardened) and the
hardened hubs and sliders with select 'fits.'
You can convert an M20 to an M22 with a new gearset AND a M22-specific reverse front idler gear. The mainshaft reverse and rear idler are used across the board.
You may see high-end units with a multi-piece cluster gear; its essentially a splined shaft that you press fit 3rd and 4th on and hold in place with din-style snaprings. This is a part of the 'M23' Program (Severe Duty) which is proprietary to us and certain distributors, but I don't want this to be an advertisement, so PM me if you want more specifics on that.
Some guys use "house brand" gears or Indian/Chinese made M22 copies. They have a M22 ratio and an M22Wide ratio. Their M22W ratio is a 2.52/1.88 1st and 2nd gear. The Italian ones are 2.56 and 1.75 for 1st and 2nd. Im not saying the Chinese stuff is bad, usually its 'okay', but don't pay premium price for Chinese stuff. I hate seeing people get screwed. If you have any question about who carries the Italian stuff, by all means ask me privately. Hell I have Asian-made gears in an M20; they look better than the factory stuff did. But, its a crap-shoot.
An aftermarket Muncie with the Euroricambi gearset and a 26 spline input is perfectly happy behind a 502/502 ZZ502 BBC. If you are going to get rough with it, I'd go 32 spline output, shifter handle be damned. If you want to try and run with the big dogs, an M23 takes up where the M22-based stuff leaves off. An M22-based, italian gearset, aftermarket 'Muncie', with the roller sidecover, iron midplate, Supercase, Super Tailhousing and 26 spline input and 27 spline output weighs about 85 pounds dry and takes 1-qt of 75w90 non-synth GL4.