I have no experience with a 1780 at all, but we pull our 1790 31 row with our 35 Challenger and I wouldn't want any less tractor or any less hydraulics.  I believe you are going to have all the same hydraulic issues related to the Rube Goldberg hydraulic setup Deere put on these things, compounded a little bit by the fact you will have a little less flow availible.  Power and traction will be marginal at best with a 7800. 

We have considered using the 75C instead of the 35 on ours.