Re transplanting the M46 from 240 to 740: you will need to swap the bell housing, shifter cage & linkage, transmission mount, possibly drive shaft + center bearing (but both donor and recipient cars must be the same body style -- both sedan or both wagon). If your 700 uses the rubber donut in place of the front U-joints then replace it with a new one: when mine failed it took out the OD's output flange; I keep it as a souvenir. Use a new gasket between bell housing and transmission. Purists will note that you should check the clearance, but many do not do this. Also take the opportunity to drain tranny oil & replace with new. Don't forget to clean the OD screen.

1-800-25-VOLVO sells M46 transmission rebuild manuals. I bought one. I am not yet brave enough to use it...

Swapping the transmissions should not be very tough. Tell us how you fare.