TH350 to 4L60E.. trans linkage?

[chevy72blu]

I am swapping an LS1 with a 4L60E into my 72. The truck currently has a B&M megashifter that was used on my previous TH350. The (not so friendly) tech at B&M told me I needed a new shifter before saying I could just change the bracket. B&M only makes one bracket for it that they call universal for all GM trans (350, 400, 700r4, 4l60e, etc).

When I shift to "D", I understand that the trans will upshift as normal into 4th but does this mean I can only ratchet to 1-2-3 (meaning it will auto shift 3-4)? I see there is also a 1-2-3-D window, but I dont understand how that works with only 3 ratchet detents. I'm sure, like most things, that I'm just looking at it wrong. Can anyone shed some light?

Also, the Megashifter is just going to be a temporary solution. Is it difficult to adapt the 4L60 to the factory column shift lever?