There was contact in a couple of places between the crank webs and the cases, at the inside of the starter drive idler gear boss in the LH case, and in a similar position on the RH, with a casting ridge that I'm guessing would be for an oil gallery. So I can't really cut either out, meaning the crank would have to be relieved at the outer edge, which reduces the most significant section of balance weight (furthest from the centreline).
The same section of crank web also hit the timing shaft on the RH side, where the later model timing shafts don't have a locating ridge. The pre 98 small blocks have a timing shaft that is shimmed between the cases to locate it, unlike the big block or 98 on small blocks where the shaft is held against the LH bearing by the timing gear.
The bottom of the 600 cylinder did just clear the web, with the underside of the piston quite a distance away. And the rods cleared the timing shaft.
And the 620 pistons I was going to use (it was going to have 750 heads, similar chamber to 620 and much larger than the small chamber in the 600 heads, which I had sold anyway) stick out a bit, but a thick base gasket shaped spacer would fix that. The 6.5 mm stroke difference between 620 and 900 (61.5 mm to 68 mm) would suggest the spacer would need to be 3.25 mm thick.
Anyway, I digress. The point of this was trying to put a late model small block 6 speed into pre 98 cases. The complete 6 speed box I bought from Ebay was from an M696. The easiest way to show why it wont drop in is with some photos.
The input shaft has a larger nut on the clutch end, like all other late model boxes. So you'd need a bigger nut and possibly to do something to the clutch hub to accommodate it. The length of the clutch snout looks to be the same from threaded end to first gear. I didn't notice any difference there, but I didn't measure them. I might do that. 6 speed at the top.
The LH end of the input shaft has an extension that goes through into the recess which the clutch pushrod passes through. As the pre 98 doesn't have the clutch pushrod passing from left to right, there's no need for the extra snout, and no room in the LH case for it anyway. It has to be removed, and I figured the drop saw would take care of that.
Not a great deal of difference here. The shaft outside of the LH bearing is 10 mm longer on the 6 speed, but you'd just turn the sprocket around to fix that. The only other thing I noticed is that, while both shafts are 25 mm diameter at the sprocket spline, the 6 speed wouldn't slide into my 600 cases like the 5 speed did. Maybe it just didn't want to be there.
The shift drum is the biggest issue, with quite significant differences.
The RH bearing area is much smaller diameter on the 6 speed (1 arrowed).
The RH end of the 6 speed drum has the 6 groove snout (2) for the sprung detente lever the later bikes use. You can see at the bottom left of the snout that one ridge is missing its top, that's neutral. The sprung detente lever at the RH end of the shift drum was introduced with the 916 from memory, and every new model since has adopted it. The sprung ball detente at the LH end of the shift drum is now only for neutral, so there is only one corresponding groove in the LH end.
The difference in the shift fork groove pattern is obvious, so clearly you can't use a 5 speed drum.
I had various thoughts for overcoming these issues, from cutting and shutting the end of the 5 speed drum onto the 6 speed drum to making a spacer ring and welding on a switch pushing lump and cutting off the detent snout with the drop saw. Attempting the shut once the cuts were made is the only way you would be able to determine if that was a viable option. If not, you have two scrap shift drums.
But you still have the issue of the LH end to deal with. The LH end of the 5 speed shift drum shows the old style detente grooves. You would have to machine these into the 6 speed shift drum to use it, which for me starts to make the whole thing much more of a hassle as I don't have a mill. One option around this is to use a non RH snout detente style shift drum from a pre 916 big block as a starting point, ie, 851, 888, 900. I'm sure I have one of these around somewhere. I did compare a 900ie shift drum, which does have the RH snout detente, and the grooves appeared to be functionally the same. They weren't as smooth flowing, with a step at each transition point, but all appeared to do the same thing at the same setting.
So I didn't get any further with this fitment.
The other issue I have with the concept is that while a 6 speed is nice, the niceness comes from the ratio spread. The 5 speed ratio spread is 2.59, the 6 speed is 2.67. The big block wide ratio 6 speed is 2.88, a comparatively much bigger change. To overcome this there is the possibility of having another gear made for 6th, either 1 tooth bigger on the input or 1 tooth smaller on the output. The 26/24 ratio gives an 8% gap between 5th and 6th. 27/24 gives an 11% gap, 26/23 12%, with a ratio spread of 2.78. That makes about a 3 tooth on the rear difference between the top gear ratio of the 5 and 6 speeds. The shape of the current teeth determines if the 1 tooth idea will fly. The input shaft gear will be the easiest to make, as the output shaft gear has a shift fork groove on it.