Totally remaned engine, new block and heads. starts poorly, lack of power. have been through all of the obvious; pump,injectors,switched computers, still same problem. Need help!

Welcome.
You'll need to supply us with a 'lil more info. Make, model, year, mods, upgrades, repairs, DTC'(s), etc.
Theres also troubleshooting link at the top of this forum, copy and paste it in your reply and try to answer them questions the best you can.

1995 K3500. completely rebuilt engine new heads and block. new turbo checked pump and injectors

You need too fill out your signature like mine or most everybody else on here, it helps us too know what kind of truck and motor and everything else truck related you have.
Next fill out the diagnostic checklist, dont skip any of it.
White smoke can be caused by a number of things but here is the first that come to my mind;
1.) Blown Head Gasket
2.) Cracked Head
3.) Cracked Block
4.) Stuck/Malfunctioning Injector (white smoke could be unburned fuel).
5.) One or more Glow Plugs not working (unburned fuel from one or more
cylinders)
This is some of many things that could cause white smoke!
Peole here want to help you out, but being this is the internet and us not being at your house you need to be as detailed as you possibly can be. Tell us everything that you can.

Has it had a scanner on it to check / set or verify the injection timing and such?

thanks for listening. i am the rebuilder of this 6.5t. It has a new block, however, bored x020, decked to a protrusion of.0085 with plus 010 head gaskets. Bored and honed with torque plate. New cyl heads,original cam, new chain, new lifters. Engine starts hard with white smoke(unburnd fuel) runs good warm but poor power. pump has been tried on test truck and workes good. Goon tech did install with remand pump, injs,.Odley, when temp sender is disconected, starts like 100%. I guess this really advances the timing and would certainly melt my new pretty pistons. I know Imlooking for needle in a haystack.Thanks again. bob

Im a bit more optimistic than diesel cash, I like to check the cheap simple stuff first.
Takin a guess here, sounds like its in "limp mode". Depending on whats going on and the PCM defaults to limp mode, the PCM can cuts boost, fuel, ie "lacks power" and may cause the glow plugs to no longer function so "hards starts" can be par for the course.

#1 priority, pull and list DTC's.

Then check:
1. Check battery condition and battery connections.
2. Check all grounds.
3. Check condition of the fuel filter and air filter.
4. Lift Pump and Lift Pump Circuit.
5. Fuel supply system for fuel/air leaks.
6. Check the gas gap isnt creating a vacuum in the fuel tank.

thanks for listening. i am the rebuilder of this 6.5t. It has a new block, however, bored x020, decked to a protrusion of.0085 with plus 010 head gaskets. Bored and honed with torque plate. New cyl heads,original cam, new chain, new lifters. Engine starts hard with white smoke(unburnd fuel) runs good warm but poor power. pump has been tried on test truck and workes good. Goon tech did install with remand pump, injs,.Odley, when temp sender is disconected, starts like 100%. I guess this really advances the timing and would certainly melt my new pretty pistons. I know Imlooking for needle in a haystack.Thanks again. bob

May be as simple as faulty Temp sensor... Smoking at all during or after start up? Have you set the timing yet?

I am not the installer but can assure you the timing has been set. From what he tells me , He just cant seem to get enough advance. Even removed front cover to verify cam timing but all looks good. Does not smoke after warm up. Runs great but poor power. He says all looks right with pump and drivers and inj system . Very frustrating!

What year is it? I have a '94 6.5 in mine and it has the exact same symtoms as yours does, hard start, sometimes I have to apply throttle just to get it to start, heavy white smoke for 1-3 minutes even if it is warm outside, and rough idle. After it warms up the smoke stops, but it still lacks power. If I unplug the temp sensor, it starts right up and runs fine with no smoke, but idles very high. Sound familiar! I have replaced the glow plugs, injectors, and the temp sensor in the water cross-over, but none of these have helped.

The weird thing is, when I got the donor truck, it started and ran fine, anytime. I pulled the engine, tranny, and the harness out to put it in my S10, but it sat for 3 years before I got everything changed over. Now it is doing all of this. I'm not in anyway trying to highjack your thread, just in the same situation.

I am not the installer but can assure you the timing has been set. From what he tells me , He just cant seem to get enough advance. Even removed front cover to verify cam timing but all looks good. Does not smoke after warm up. Runs great but poor power. He says all looks right with pump and drivers and inj system . Very frustrating!

Im curious. Exactly how did you set the timing on a '95 6.5 electronic (OBD-1) if the engine was not in the vehicle and running at operating temps at the time??