You might want to consider the MD20
RZT high head/pressure model, especialy if you plan on multiple blocks(GPU&NB) and a lot of tubing. it's better to have a pump that will maintane 3+GPM than have one that'll drop more unless pumping 'wide open'...
I'd like to see them both tested to see how well the RLT holds up (GPM wise) when restriction is added, compared to the RZT, cause I'm only guessing at the mo' ...
PS, I believe the only difference between the RLT&RZT is the 'impellor' and 'impellor housing', it might be worth ordering a different model replacement impellor&housing if possible for testing and optimising the setup. I certainly will if I get round to saving up the £165 it costs for a 20 series IwakiMD in the UK

, for that price I'll have to look into ordering from Stateside...