How does it run currently? Can you go freeway speeds? I am going to assume the GMC is a truck? Is it a half ton?

If I remember correctly the guys running the Myna pumped diesels where using HX35's. I don't know how much boost is safe, but I think your head gasket may be in danger over 14 psi or so. You may be in for some upgrades, and I am concerned they may hurt reliability. I sure am curious how it runs stock and if it is close to livable.

 

i plan to run 20 psi max im working on the swap still motor is siting on a tire the motor ran great no blow by at start up and none once heated up it is a half ton truck it had 4.3 vortech motor in it it pulled my camper quite well. i just think theres no way 25 year old turbo would be the best for this maybe a larger divided flange for less lag and better flow

 

Unless you've got some secret fuel pump mods planned, a bigger turbo won't be any use. With an intercooler you will get more air than your fuel pump needs.

 

im going to turn the pump up http://www.superturbodiesel.com/std/ip-full-load-adjustment-procedure-for-mw-pumps-t-189.html i want a longer flatter powerband with low egts

 

There's very little left in those pumps by all account. Turning the screws will only get you a few more hp.
But a variable vane turbo will flatten out the power band by bolstering low and high end compared to stock.

 

http://www.4btswaps.com/forum/showthread.php?14262-Long-runners-on-a-turbo-diesel-make-no-sense