DON'T PANIC. Piece of cake!
Greetings and Hello, Red Merlin of 2600 sprint restoration fame in merry old England! I admire and applaud your desire to actually turn nuts and bolts on your own project. You can do it!!! Forgive me for a completely politically incorrect story my first mentor told me the first time I hesitated when I had to attempt something new (for me) on my first Alfa years and years ago, "You just have to assume the right attitude of superiority", he said. "Look at your self. You're smart, well educated, and well fed. Just think, the guy who put that Alfa together couldn't even speak English". It struck me a such a stupid nonsensical comment I almost split my sides laughing. What difference did that make when I couldn't speak his Italian either. But since then whenever I have to tackle any new problem on any of my cars I can't help recalling that comment of my now long gone friend and smiling. That was fifty years or so ago.
Anyway, relax! Just dive in. Transmissions are not all that difficult, and I am glad you made it clear you were dealing with a MID SEVENTIES 2000 BOX (i.e. NOT a two liter, 102 series box which has different number of teeth in input shaft and unworkable except by using whole box and a Giulietta output flange to fit greater number of splines and still have three bolts instead of regular four bolt of two liter). YES, the switch is simple and I expect all the parts of the "newer" box will be perfect (but do check the teeth, the wear, etc), so: (1) Removing rear nuts off tail shafts might take a "rattlegun" air wrench on both your 2600 box and also the new donor box. (2) Remove the rear tower sections on each by removing the nuts to rear, putting the shift mechanism into third gear to free it from the shift rail, and lifting it up as you remove it. (3) Take apart the clam shells -- held together with bolts and nuts only. (4) Look carefully and compare the pieces inside of both. VIOLA! The guts from the mid seventies box fits right in place of the 2600 guts. Might have to loosen the fork that holds fifth and reverse to fit them together.
HOWEVER, (4) The mid seventies box is equipped with Moly synchros instead of the old type, so you must make the one on the 2600 input shaft conform (might also check condition of the bearing) -- but then rejoice because you can now forget the need to use Dentax 90 as was necessary in the old boxes. (5) On reassembly be sure to use the 2600 end piece (the one that goes through the real seal and has the three arms for the bolts on the guibo (i.e. metalastic or rubber donut -- whatever term you use) on the new replacement innards so the speedo drive for the 2600 will match the pickup in the shift tower. Don't mix and match these if you want your speedometer to actually give you some proximity of the correct mph or kph (I believe that all have a built in 10% optimistic factor anyway). Besides, if you mismatch the mid seventies speedo driver with the 2600 pick up in the shift tower (or vice versa) you probably end up galling the softer gear and lose your speedo drive entirely. And if you use the end drive of the newer box with the shift tower of the new box too, the speedo will work but be way off because of rear end ratios and tire size.
So, you can do it!!! Use some good sealant on the surfaces to avoid leaks. Not rocket science.
Size of seals: Input in bell housing is standard Alfa # 2340.34361D and Output in shift tower standard Alfa # 2340.34406D from two liter parts book. (But these seals are consistent for all the boxes, so numbers for cars such as spiders from 1967 through 1993 fit. Sorry I can't recall for the moment the actual metric sizes, but if you take yours out any good bearing shop can measure yours and give you equivalents).
Okay, once you put the silent blocks in, use a bolt to screw just into the block to hold it, but put a lock nut on the bolt to jam it fast onto the bellhousing.
P.S, I am still interested in the your inventive adoptive but non suitable bellhousing to fit the old five speed 2600 (or two liter) tranny to the rear of a v-6 alfa engine. I realize that using the v-6 in a 2600 or two liter might require cutting the front cross member, bit I wouldn't be using the old style front suspension in the wild idea I am dreaming up. Once you have everything out of it, let me know your price. And I have a good 2600 driveline for a spider in exchange -- but you have to verify if the length is right for a sprint. I believe it is, but I want you to verify for yourself. Perhaps someone on the BB can verify one way or other. Jay in Seattle.
[B]JAY NUXOLL [/B][EMAIL="firstname.lastname@example.org"], seriously Alfa diseased and ancient OLD Two Liter Lover, put together Seattle area's Northwest Alfa Romeo Club in 1965, and still feebly tries to tend a teeny sacred flame to his serpent mistress in the [B]ALFA G'RAJ MAHAL[/B], a home garage temple with more Alfa cars and parts than he dare list because of the disapproval of his shamed and chagrined family. (425) 641-2600.