Laydown close ratio question

Back in 1973,     I purchased an Atlas gearbox that had been modified in a similar way. The result being that the top 3 gears were very close but 1st was a huge wheel locking drop below 2nd. 

The resulting AMC tooth count of my box being:-

Mainshaft - 14  20  22  23 

Layshaft  -  28  22  20  18

This set-up allowed me to keep the use of a kickstart.  Apparently the Laydown Boxes were easier to modify to close ratio with just a little machining required but don't ask me what was done.   Standard Laydown box tooth count:-

Mainshaft  -  24  22  18  13

Layshaft    -  18  20  24  29

AMC gears won't fit Laydown (or dolls head) because the earlier gears for 2nd and 3rd linked with 4 dogs, and AMC has 3.  Can someone confirm if the AMC 2nd and 3rd can be substituted together?

First gear AMC will fit but the later first (small) mainshaft gear has an end boss that needs turning off.  

RGM (and probably the others) has most gears in stock.

The highest first gear available is the Daytona pair, which is available for all years.  Layshaft (larger) first fits all boxes from Doll's head.  Mainshaft comes in two variants...with and without end boss.

Dolls head, Clocktower Laydown and AMC gears will interchange between all boxes BUT they must be paired' AMC 2nd Pair will fit all others. All other 2nd gears paired will fit AMC. But you can't mix and match individual gears.