K-Var has the Krinkov gas blocks and a slightly larger combo type. Both gas blocks have 90 degree gas ports. This means you will have to drill a new gas port to use either one with the AMD-65 barrel. The larger one is shown in the following pic:
http://home.woh.rr.com/rnystrom/receiver-barrel_stub.jpgThe barrel shown in the pic is a 5.45 barrel stub from K-Var. The gas tube is one that I modified to fit standard handguards to one of my AMD-65 kits.
The AMD barrel is longer than the one in the pic. If you use the combo gas block, the AMD barrel gas port will wind up under the threads of the block. If you use the Krink gas block, the AMD gas port will be just ahead the threads of the block. This means you will have to cut the barrel at the gas port and recrown it. Leaving the gas port in place could damage the gas block or redirect hot gas back towards your face.
Ideally, you should have about 1" of barrel between the gas port and muzzle as a minimum for proper operation of the gas system. This was the distance that I was told would be preferable by someone who has built several SBR versions.
You could swap over to a Krink gas tube and gas block, but that will create clearance problems with the AMD handguard retainer. You would also have to change over to a Krink handguard retainer. The Krink HG retainer notch looks to be closer to the receiver than the standard HG retainer notch. This means Krink furniture as well.
Swapping to Krink parts will not be cheap:
Krink gas block $90
Krink gas tube $30
Krink HG retainer $40
Krink furniture $70 to $110
It's do-able, but it won't be cheap.