I picked up a new safe on a black Friday sale and need to reenforce the floor before putting it in place, The question is whats the best way to do that?

safe foot print 28 x 50, 880 lbs

joists are 16 in centers and is being placed against a load bearing wall, center of house with pillars and beam support under the house

Planning on cement pier blocks with trailer type leveling jacks holding a 4x6 , don't know if I can get a 6 foot 4x6 under the house to be able to support 4 joists or if a 4 foot 4x6 and only catching 3 joists will be enough? Planning on 2 spots of support front and back of safe

You must have a stash for that big safe. I have no opinion or options for your restructuring. I have one small 2 x 2 foot that suffice my needs.

How much space between the bottom of the joists and the ground?

Maybe 2.5 feet or 3 to the bottom of the joists

My son in laws that have to help move it think I should get 2 smaller ones lol Need one this big so the guns have room to reproduce and multiply without the wife knowing lol

Design live load (things that you can move) is 40lbs prer sq ft. Your safe is 110 lbs per sq ft. A lot more than design load but not enough to fall through. Design load is complicated. It's for an average load of 40 lbs per sq ft. If you set your safe in a place where there aren't other very heavy things around it, the beams contributing to supporting the safe can still be fine if the total load on that beam is still no more than 40 lbs per sq ft. It's even a little more complicated than than because weight at the center of a span is different than weight near the end of a span but I'm confident that you're okay. When the code is written, they know somebody is going to set a gun safe somewhere. If I was worried, I would measure the height of the bottom of the bean unloaded and then set the safe and measure how much deflection there is. Code wants only 1/240 of the span in deflection. So if your beam is 240 inches long, it's allowed to deflect 1 inch. If the deflection exceeded L/240 (it won't) I might care. I probably still don't because, unless my pool table is crooked, deflection doesn't matter much to me. I care about catastrophic failure and at 880 lb point load you're fine. We have people that heavy who walk around their houses (or roll around) every day and they don't fall through.

I believe the joists are only 14 or 16 foot long from the outside wall to the center beam, and its a spare room with just a dresser and bed in it. If I make sure it spans 3 joists and its at the end of the joists that should also help with the load correct?

Retired General Contractor for 40 years here.
First thing: What size are your joists? A 2"x10" holds a lot more than a 2"x8".
Also, 3/4" sheeting is stronger than 5/8" between the joists and spreading the load. Just because you are over 3 joists doesn't mean the next joist over on either side of the loaded joists aren't doing a bit of the "carrying".
Think of a Baby Grand piano. The load is resting on 4 legs, and likely not directly centered on a joist. Rather, the sheeting is spreading the load to 2 joists.
I have fortified many floor systems using 4"x6" material and posts when adding tile to a large floor, for instance. While I wasn't concerned that the floor system could handle the additional weight, (a single box of floor tile is heavy!), I was concerned about floor deflection, resulting in cracked grout joints. I would typically use pier blocks with 4"x4" posts under the 4"x6"s every 4' or so.
For your situation, I would be comfortable with a 49-1/2" 4"x6", (that carries all 4 joists "out to out" in construction lingo), or even 4"x4" "beam", and 2 4"x4" posts resting on pier blocks.
800 pounds, plus whatever you put in it, isn't that great a load when spread out over a floor system that has ground support nearby.

I'd add something solid on the ground like those post blocks and then cut some some boards to length a fuzz long and pound them in putting the safe on stilts. Maybe use 2x6 or something. Maybe add a runners (headers) across the bottom of the joices. Depends on how much room ya got I guess.

really depends on what type of joist you have. I have built a couple homes but am not an expert. are they the manufactured joist? or 2x10, 2x8? I would trust the solid wood more than the glued up ones they use now. if solid wood add a board next to them, under house work is no fun.

wws

You’re getting good advice above. I’ll add that you need to change “cement” to concrete. Especially if you’re talking to a contractor for a bid. They won’t take advantage of you so bad.

I'd double check on the joist hangers, make sure they aren't just toe nailed in. If they have good galvanized hangers, I'll bet you'll be just fine. Most people wouldn't even think to look and just go set it in place, like me.

This should work just fine, more than most would do.