Shortround6
Major General
The R-2800 appears to be about 300lbs heavier than the R-2600 but I suggest it is within the capacity of the A20 to handle especially as the bomb bay would not be used.
The two stage supercharged engines in the P-61 were 550lbs heavier than the R-2600s. the Single stage engines used with turbos were only about 300lbs heavier but then you need the weight of the turbos, the ducting, the larger intercoolers. You also have the larger, heavier propellers, without which there isn't much point in putting in the fancy high altitude engines.
And again, design work was being done on the A-26 through most of 1941. Doing design/engineering work on an R-2800 powered A-20 is only going to delay the A-26.