I used Matsushita 6DJ8's in my 5i with good results. Do a search here and you will find they are good tubes and priced reasonably. I still have 4 of them. A friend of mine is using 4 of them to see if he likes them in his Audio Research preamp. I found them to sound better than the stock Sovteks thats for sure. The 5881's I recommend are Tung Sol. If you decide to buy a 5i change out the 5881's 1st and then proceed from there.
As far as heat, I ended up moving my 5i to the 2nd shelf and never had any heat issues or excessive heat on the shelf above. I think the distance was about the same as yours or maybe a tad taller.
As far as heat, I ended up moving my 5i to the 2nd shelf and never had any heat issues or excessive heat on the shelf above. I think the distance was about the same as yours or maybe a tad taller.