It had already been decided that he Panzer Mk IIII was to be up gunned in May 1941 and would have been up gunned to 75mm soon after. German tank development was not just a result of war in the Soviet Union. The Germans had already realised their tank guns to be inadequate against the British tanks in Libya. They will have even more experience here while facing the British in the Middle East.
The Pz III 75mm only came into service in 1942-1943, There where only 700 of them and considering there lower capabilities to that of the Pz IV there fielding would not have made that big of a difference. Even then they had to be up dead close to get any real penetration as the 75mm gun was only an L/24 giving very limited range for effect. German tank development was not just a result of the Eastern front, agreed but it was the main contributing factor that led to the massive increase in the R&D and production.
Make of this what you will !st Panzer Division, Fall Blau: 28 June, 1942 Pz II 2 Pz 38t 10 Pz III 26 Pz IV 10 2nd Panzer Division, same date Pz II 22 Pz 38t 33 Pz III 20 Pz IV 5 3rd Panzer Division, same date Pz II 25 Pz III 106 Pz IV 33 3rd Panzer Division, 1 july, 1943 Pz II 7 Pz III 59 Pz IV 23 4th Panzer Division, 28 June 1942 Pz II 13 Pz III 28 Pz IV 5 5th Panzer Division, same date Pz II 26 Pz III 55 Pz IV 13 6th Panzer Division, 1 July 1943 Pz II 13 Pz III 52 Pz IV 32 FlamPz 14 7th Panzer Division, same date Pz II 12 Pz III 55 Pz Iv 38 8th Panzer Division, 28 June 1942 Pz II 1 Pz 38t 65 Pz IV 2 8th Panzer Division, 1 July, 1943 Pz II 14 Pz 38t 3 Pz III 59 Pz Iv 22 9th Panzer Division, 28 June, 1942 Pz II 22 Pz III 99 Pz Iv 21 9th Panzer Division, 1 July, 1943 Pz II 1 Pz III 38 Pz Iv 38 10th Panzer Division, December 1942 (North Africa) Pz II 21 Pz III 105 Pz IV 20 11th Panzer Division, 28 June, 1942 Pz II 15 Pz III 110 Pz Iv 13 11th Panzer Division, 1 July, 1943 Pz II 8 Pz III 62 Pz Iv 26 12th Panzer Division, same date Pz II 6 Pz III 36 Pz IV 37 13th Panzer Division, 28 June, 1942 Pz II 15 Pz III 71 Pz IV 12 14th Panzer Division, same date Pz II 14 Pz III 60 Pz Iv 24 15th Panzer Division, 25 May, 1942 (North Africa) Pz II 29 Pz III 134 Pz IV 22 15th Panzer Division, 23 October, 1942 Pz II 14 Pz III 87 Pz Iv 18 16th Panzer Division, 28 June, 1942 Pz II 13 Pz III 57 Pz Iv 27 17th Panzer Division, 28 June, 1942 Pz ii 17 Pz iii 36 Pz Iv 16 17th Panzer Division, December, 1942 Pz II 4 Pz III 29 Pz IV 32 18th Panzer Division, 28 June, 1942 Pz ii 11 Pz III 20 Pz IV 8 18th Panzer Division, 1 July 1943 Pz II 5 Pz III 30 Pz Iv 24 19th Panzer Division. 28 June, 1942 Pz II 6 Pz III 12 Pz 38t 35 Pz Iv 4 19th Panzer Division, 1 July, 1943 Pz II 2 Pz III 38 Pz Iv 38 20th Panzer Division, 28 June, 1942 Pz II 8 Pz III 20 Pz 38t 39 Pz IV 13 21st Panzer Division, 25 May, 1942 (North Africa) Pz II 29 Pz III 134 Pz IV 22 21st Panzer Division, 23 October, 1942 Pz II 14 Pz III 87 Pz IV 18 22nd Panzer Division, 28 June, 1942 Pz II 28 Pz III 12 Pz 38t 114 Pz Iv 22 23rd Panzer Division, 1942-1943 Pz II 27 Pz III 84 Pz IV 17 24th Panzer Division, 28 June, 1942 Pz II 32 Pz III 91 Pz IV 32 Source Wehrmacht Panzer Divisions 1939-45, by Jorge Rosado & Chris Bishop.
I see that BE does not know the difference between single track and double track ,I expected nothing else . About Spain : the economic aid :Germany would not be able to give it,of course,I expect the reply :nothing would be impossible for Germany . the "possibility" for a German army corps to go (in the winter)from Biarritz to Gibraltar (or via the southern route):the distance Biarritz Gibraltar via Madrid is 1150 km,along the coast some 1400,via Madrid is of course impossible,even today it would be dubious . Now,as BE has no notion of it,some questions : how long would it take 60000 men (mostly obliged to goose-stepping),400 tanks,400 pieces of artillery and 3000 motor vehicles to go to Gibraltar ? A hint :3 months would be close. how much road space would they need ? (some one could explain to him what road space is ?,but very elementary) 400 km would be a minimum what would be the condition of the tanks,artillery and motorvehicles,when they would arrive at gibraltar (if would be a better word)? A hint (I am generous today):ready for the old car dump. and the condition of the men : not better. And,of course,he never has done some thinking on the question how heavy,very heavy artillery could be transported by road,I don't think that there were prime-movers that strong.And,what would happen with the bridges (a lot of them temporary ones) could they carry a tank of 30 tons ? Etc,etc About the possibility to do the whole thing by rail:the Spanish railways had (and partly still have) a different gauge from the French ones (some one could explain him what a gauge is ?),and,how many trains would be needed to transport 60000 men, 500 tanks,500 pieces of artillery,3000 motor vehicles,ammunition,fuel,..? (I will not give a hint:my generosity has its limits) Of course,we know :nothing is impossible for the Germans ....