Yes, should be a 5 round mag but will probably hold 6 anyway.

The Brnos had a better quality of timber/bedding than the later CZs and probably won't need the cross bolts, but I guess it wouldn't hurt to add them. Hopefully it doesn't have the jamming issues (for those rounds that come out of the LHS of the magazine) - I believe that even in the Brno days you'd have a 50/50 chance of getting one that didn't jam (in 458 only).

You probably also have the flip-up peep in the rear of the action.

You'll probably also want to change the recoil pad - those hard rubber ones don't compare to the modern versions.

I had one of those in 375H&H many years ago and it shot MOA consistantly straight out of the box, but I guess this is not a requirement for a 458 which is not intended for any long range hunting.

Well done on the price. When I was looking around to buy my 458 (I wanted a Brno) they were few and far between and didn't stay on the market for long. This was about a year ago and prices ranged from $900-$1,200 bare rifle. I saw one at $900 but it was non-standard and a bit worn (cosmetically) here and there. A second-hand CZ came up at that time and I got that instead.