Here's a shot I've noticed ya don't see too much with the DIY tanks, the inside - particularly the mounts... there was much
discussion and debate regarding those mounts, the backing plates and their size my idea - their thickness his, neither of us thought it a good idea to just weld nuts/studs to the tank itself as the stock tanks have, for obvious reasons. (click to view larger);
You'll note the perhaps overly thick backing plates for the mounts, the thickness just to help make the welding easier - they're also not welded completely around their edges. While this would ofc be desirable, it's not particularly needful as I won't be relying on them to seal - they'll have studs sealed on the outside w/a nut... the thought being for strength and having lots of material should I ever need to retap them. All the internal fasteners, fill cap, and backing plates were MIG welded to help "build up" the material. The tank will be lined ofc.
I do feel the need to mention that this fastener method was experimental, the tried & tru technique is welding acorn nuts on the inside. Provided they're welded completely around, they won't leak. Weld the acorn nut to a large washer first, then welding that to the tank helps prevent the stud from bending/distorting the tank.
Before the bottom was tacked in place - the tank got a ride home again for one final test fit. Much as I tried, there was never a chance to get it all in one piece, accurately - with nothing but mere tape. To say I was happy to note a near perfect fit and uniform frame spacing would be an understatement heh
Back to work again, the bottom panel tacked in on a lunch break - it now sits on a shelf gettin' dusty as Murphy has his way with us...