There are many 7mm GC molds out there, all it takes is to ream or bore the GC off the mold. There is also a occasional non GC mold listed on Ebay but, you can't be impatient. Look for the 287202, 287221 or the 287373. They won't be cheap @ $75 - 100 per mold. GC molds that are more common ($40 - 65) and can be modified to PB are 287405 and 287346. If you do that, make sure that the base band is about .002 over groove with .0015 added for alloy shrinkage.

Frank

CPA stocks a 7MM plainbase mould for a 284 Groove, not sure if that is helpful not knowing all the specifics on the barrel.

I've used gas check bullets without a check on them and they shoot well depending on bullet and if the rifle will shoot accurately anyway. I shot the Forsyth 1000 yd match in 2003 with a 640 gr spitzer 1 grease groove 2 driving bands bullet the gas check base had lube. My first 5 shots could be covered with your hand @ 1000 yds, then I started leading up, wrong lube and 114 in the shade and no shade.

I have been reading the old original Single Shot Rifle News on disc. (If you are really curious about why things are the way they are get one from Byron Goff.) In the first match in 1948 there were 2 classes plain based and gas checked. Only one person showed up and withdrew because there was no one to shoot against. Later when set rules were established the rules stated that only plain based lead bullets would be allowed. 
These are the first shooting rules published for the first match:
SHOOTING RULES OF THE AMERICAN SINGLE SHOT RIFLE ASSOCIATION
l. Any single shot cartridge rifle as such - no magazine type rifles with alteration for single shots.
2. No barrel length restriction - weight limitation 20 pounds.
3. Rest shooting to be done from bench or prone position, use of sling permitted.
4. Single case loading and firing permitted.
5. Telescopes to be permitted in any sight matches.
6. No restriction on type of powders.
7. Matches will specify type of bullet which can be fired either lead only or gas check and metal jacketed. Type of bullet designated MUST BE USED ONLY IN THAT MATCH.
8. The use of card, felt, or other non metallic wads shall be permitted except wads containing asbestos which will be classed as metallic gas check.
9. Case, breech or muzzle loading seating of bullet, optional.
10. No caliber restriction.
ll. Scores to be computed from center of hole. Scores and targets to be witnessed. Shooters will furnish the fired targets together with load, bullet, type of rifle, etc.
12. Targets to be "Official" German ring targets only - only scores fired on targets signed by Secretary to be considered.
13. Targets must be fired at ranges specified and can be fired at any home range. All firing must be witnessed.
14. Only dues paying members will be eligible to compete for prizes in all matches. Dues are $l.00.

Continued:

Ah, that clears things up nicely.  Thanx.

At that match I used Lee liquid Alox dipped them into it and set on wax paper to dry overnight then dipped them again,and dried again, I've been using a RCBS luber with a longer I die or ram to push the bullet completely out of the sizer, the lube will stay on the gas check part of bullet.  I use Lyman Molly lube. Black nasty stuff but works well to stop leading.

I agree that the base of the bullet is one of the most important things with a lead projectile.  Good cast bases are very necessary. (this includes the sprue area on base pour bullets).
I shoot only bp and I use a .060 veggie wad to protect the base.   
I have also had my guns recrowned so there is an absolute 'sharp' edge to the crown that is square to the bore (last thing the base sees).
Getting these three things right is one of the things paramount to accuracy with 'pb' bullets.

Cary