If you really want to lock out your HEI by welding it then simply remove the weights and springs and weld it. One of the reasons that your timing retards as RPM increases is the fact that there is a certain amount of "Dwell" built into the module hence the reason for centrifugal advance. You really should toss the module along with the weights anyways and get yourself an aftermarket ignition box and fire it off of the magnetic pickup.
After you weld the distributor you should make sure it is phased correctly. Pay someone to put it on a distributor machine. It can also be done in the car but it takes patience.