The NMRA recommended minimum radii is here:
http://www.nmra.com/standards/rp-11.html
The Layout Design Special Interest Group rule of thumb is a minimum radius of 3x the car length (measured over the coupler knuckle faces) for reliable tracking; 4x for improved appearance; and 5x for automatic coupling to work reliably. The rule of thumb assumes 2 of the longest cars coupled together with body-mounted couplers. The 3x rule for 89 ft cars would give about at least a 36" minimum radius for the helix, and ideally 48" out in the open.
You could probably get away with less as others suggested by using truck mounted couplers. In my experience, you can get generally get away with about 2.5X length for minimum radius, but the tolerances for misalignments in track, misgauging or wobbling of wheels, coupler mountings that are less than spot on, etc, all become much smaller. Also going below the 3x recommendations sets you up for string-lining with long trains (especially in a helix), and of course pretty poor appearance. Less than 2.5X can be done but requires undeframe and detail modifications, and truck mounted couplers. Long trains don't work.
Due to the growth in car lengths and train sizes modeling modern era in HO convincingly requires much bigger spaces than we are used to thinking of. If you do the math, modeling modern era in N takes just as much space as modeling transition era in HO.
my thoughts, your choices