the 5150 shares much of the same internals as it's rebuildable 7100 brethren. They can be revalved, yes, but the issue is getting it recharged. I think someone on Pirate had welded a shraeder valve onto their 51** body and got 'er done, but i kinda doubt this is worth the hassle.
If you haven't dealt with Shaffers before in NV it's worth a call. I ordered the 7100's for my rangie from them, and they were very helpful. They also work directly with Bilstein out of LA.
If I remember correctly, they were able to get custom valving done direct from Bilstein for the 5100 series. You might be tempted to buy 7100's and revalve them yourself - the process is straight forward, but there's more equipment you need to buy to do it right, and it's a PITA. If you can get the Serious I down to a good scale and get corner weights loaded up, take those numbers back to Shaffer and they should be able to get you fixed up with the right shocks.
my two bits -
-isaac / searover
gas > mods
D1 equal parts honkey tonk and slayer
1977 Chrysler New Yorker Brougham. Factory Sure Grip 8.75 axle. 440 big block. 4 body trunk. Tikki torches, picnic blanket, martini kit. Discount coupon stickers to the Love Shop. Refurbished hood ornament.