Does the remington 798 in 458 win use the same action as thwe 375H&H,3.6" length??Can pick one up at a very resonable cost, but would rechamber to a 458lott or 458/375 ultra mag.

The 375 & 458 use the same action different magazine box. Any 798, Daly MKX use the same action. Magnums have the bolt face altered and the 243-308 have a spacer in the back of the box and shorter follower. To make a normal magnum work on the Lott work will need to be done to the box as well as the action. A 375 box is thinned in the back and extended in the front. The the action feed ramp is shortened to match. A 375 action would only need the barrel swapped. A 458/ultra would probaly require some additional work.

Many shy away from high pressure long magnums in a MKX (798) because of the amount of metal removed from behind the bottom lug.

Before I would spend any $$ buying a converting a 798 to a Lott I'g guy a CZ 550 and just go shooting.