The P51VLR 20mm hispano bird and the P51A are totally different birds. The VLR was a poor A2A bird and was forced into ground attack role and even then the British preferred other birds over it. The P51A i wouldnt mind though even if it wouldnt be the best of fighters 

Ah man...Corky's P-51VLR thread snagged yet another one. I thought he had pulled up his bait long ago but I guess he must have forgotten a hook in the water.
Anyway...the Mustang Mk Ia were the RAF 20mm cannon equipped Mustangs that were pretty much used in the Army co-operation role (i.e. ground support and intruder missions) and recce duties.
Okay...the P-51 VLR that Corky had posted about...it was a parody of Chalenge's wish to get larger drop tanks for the Mustang and all the wishes posted for a cannon equipped P-51. In reality, the "VLR P-51s" were nothing more than P-51Ds used on VLR (
Very
Long
Range) missions. The VLR missions were primarily escort duties for the B-29 raids but as time wore on, the Japanese were less prone to engage the B-29 during the daylight raids and would avoid the Mustang escorts. To counter this, the P-51Ds flew both escort and low altitude fighter sweeps, like the P-51Ds had done over Europe.
The P-51Ds would take off on their VLR missions from Iwo Jima, with flights lasting up to 8 hours as they flew to Japan and back. The Iwo Mustangs weren't specially modified, didn't carry any special armament or cannons as they were just normal P-51Ds.
This is page is the history of the 506th Fighter Group and tells the story via pilot's comments of the first VLR raid by the Iwo P-51Ds. It also tells the story of a banzai attack one of the 506th squadrons beat back.
506th FG - First VLR to JapanThis tells the history of the VLR missions.
VLR Historyack-ack