integrating new & old piping

Hello smart people—I'm in the process of updating an old house, one bit at a time. I've transitioned to a new boiler and some radiant in-floor heat, with more to come. However, I'm exhausted and broke, so I'm going to leave 1 part of the remodel for later. Existing heating consists of about 58' of 3/4" fin tube, fed by 1-1/4 copper pipe. It's all buried in the floor, so I don't know the exact piping layout. I'd like to connect this to new boiler, but not sure if 3/4" manifold fitting will allow sufficent flow?

Specs: 58' slant fin baseboard and wall units @ 430 BTU/ft = 24,940 BTU (160 deg H20). 3-speed Grundfos pump.

Hoping to re-pipe with 3/4 HePEX. Or 2 lines if necessary. Eventually heat calc calls for 7 zones of 3/8 PEX for total load of 26,398 BTU.

Thanks, in advance, for those with more knowledge than I.