If you already have a built Rb25 bottom end, try and get an RB25det Neo head.
A lot cheaper and it has solid lifters
I currently run an RB26 Tomei Type B cams in my 25 neo head at the moment, 260/260 In/Ex 9.15mm Lift
I had to remove the VCT for this to work and use adjustable cam gears
Anything bigger than 9.5mm lift and you'll need to machine the head.
I've never heard of putting a 26 head on a 25 bottom end