Here is the web site for the GMAC spring. Just toss the brass rod and exchange the Crosman spring with the new GMAC spring. About $15 USD total.
The exact size of the drill bit is not critical, as long as it is larger than the diameter of the existing transfer port hole. You can't get it too large, as the valve port controls the amount of air released. All you are doing is giving that amount of air an open pathway to reach the rear of the pellet. A perfect hole is not needed. I used a AA battery powered hand drill and held the transfer port in my fingers while I drilled it. Further, the barrel port, just above the transfer port, can be drilled out with the same drill bit used on the transfer port. That way max air quickly gets to the pellet from the valve.
Some hammer spring throw and tension adjustment may be needed. Just check the A team's post on adjustments, or check Youtube for several posts on adjusting these two items.
Google an exploded view of the Prod, and you will see the four bolts holding the breech on top of the transfer port. The spring is also quite simple to get out and replace, just look over the exploded view carefully to see what screws to remove.
I used 18.1 grain JSB, which I believe are the best pellets available for the money. My velocities were 750+ fps with this pellet. An amazing increase with such a simple modification.
I was getting three magazines of eight shots with the mods. Plenty of power. Stupid simple.
I got a Jim Gaska butt stock adapter and a collapsing stock from Tipman, each was about $30.00 and made a dandy folding carbine. Jim also has a shroud extension to quiet down the noise to near nothing.
Have fun. Go slow. Enjoy. Let us know how you do.
H. E. Potter