I have a 28 Nosler that is a Bighorn TL3 (which is rem 700 foot print) with Manners EH1 with CIP length mini chassis, using accurate 3.850" mags, and COAL of 3.720". I have zero issues with feeding. Shoots and feeds great.
If you are using a center feed magazine, like AICS, I don't think you would need wider rails of Ultramag action. If using like a BDL mag, you may need them, I am not sure. But there are plenty of factory rifles chambered in 28 using BDL mags with standard long actions.
The TL3 action is CRF, I don't know if the R700 being push feed would hinder feeding properly or not with that long of a bullet.