truck system

I'm looking for guidance on replacing part of  a factory system in a '99 GMC stepside. What I am considering is:

Retaining the stock Delco head unit. May swap out later. Replace existing speakers with:

Doors: JBL GTO627 (6 1/2") 2 ohm, response 55-21000 hz.

Pillars: JBL GTO427 (4 1/2") 2 ohm, response 90-20000 hz

Being 2 ohm resistance be a problem? Of coarse the write ups on these speaker claim not.

Add: Alpine SWS-1043D (dual 4 ohm) or perhaps a 12" sub powered by either a 300 or 400 watt MTX mono amp. Sealed enclosure (custom) sitting between the bucket seats firing forwards (?).

Yes I know this is not a premium set up but part of the existing system had been blown out prior to purchasing the truck. Just trying to get it to the point where it will sound pretty good and can be enjoyed at 70 mph. I am kind of sold on the sub and amp. I have a 10" Type R with a 600 watt MTX in the trunk of my Grand Prix and it's great. I'm assuming not quite as much power is required in the front of a std. cab pickup.

Opinions and suggestions would be greatly appreciated.

I'll just give you an opinion of what I might do in that situation.  Instead of using the pillar locations I would use a tightly grouped 2-way component system woofer/tweeter in the lower door stock location (6.5" woofer is a good idea).  Keep the drivers as close to each other as possible and experiment with tweeter aiming.  Try to allow for the possibility of adjusting the tweet after install is done and several listening sessions are behind you.  You want a good bass transition in the 80 Hz range with the sub, so fortify the doors' sheet metal with damping (e.g. Dynamat).  This is a critical area of coverage if you are to enjoy sounds at 70 mph.

Buy a high quality LOC to mate your existing OEM deck with an aftermarket two-channel amplifier to power the components.  The LOC has adjustable gains.  I would not attempt to use deck power as it will be entirely insufficient.

I would also build a mock sub box that can be rotated up, down, toward the rear (etc.) for the console.  Once components and damping are installed and power is in place, you should find the best sub sound through trial of positioning.  Best sub sound is that which blends best with the door woofers with the least delay; you will be listening and testing for impactful bass response.  Once you find the best position to fire the sub finish the cosmetics and mounting of the sub enclosure, but be sure to secure the box to the vehicle throughout the trial period.

OEM wiring should be beefed up to get the power to the amplifiers, but you might find it easier to use some of the existing speaker wire in the doors.  I would try first to run new wires and it can usually be accomplished.  There are several threads that have been stickied on this forum that will help you out if you haven't read them yet.

Thank you for your reply. I've watched this forum enough to know your suggestions are based on more than just opinions. Not knowing a great deal about "component" systems, though, has brought up some more questions from me.

"Tightly grouped"  implies there is more than one speaker per side (other than the tweeter). Suggestions on size and range for the second speaker set? Should I automatically eliminate 6 1/2" mid's from consideration that have a smaller speaker built on to it? Actually that is what I thought a two way system was. Are not some components built this way plus the seperate tweeters?

Does someone have knowledge of the RMS wattage of the stock Delco head? (basic CD and AM/FM radio) What kind of power would you suggest for the 2 channel amp? The inside of the cab isn't all that loud as far as road noise goes. Of coarse it does mask some of the higher frequencies.

I am intending on replacing the speaker wires especially if I go to an additional amplifier. And thank you for the suggestion of making a mock up sub box to maximize it's performance.