not to many problems the
RB20 is rated 215 BHp
RB25 is rated 250 Bhp
Rb26 is the grandaddy of them all 280 Hp

all of them are straight 6
to install it into a S-13
you need eather R32 crossmember
or a set of mounts from eather Top hat or mckinney
the RB25 you need a driveshaft made as well
the RB26 you need a custom DP if you want to keep twin turbos
RB20 Tranny for 400 HP RB25 Tranny for more like 700 Hp
and a Rb25 oil pan plus a drive shaft.
then just the wiring harness and if you want a tach for a 6 cyl