I read on the forum you could put a 5/8th" grade5 bolt through the axle to strengthen it.I've not heard of it or tried it. Two issues to watch are
(1) Extra parasite drag down low, where it tends to pull the nose down at high airspeeds.
As long as the high speed is in level flight, the throttle setting will be medium-high, powering up the H-stab and lessening the chance of drag-over. In a power-off dive, OTOH, the centered H-stab receives little propwash, and is noticeably less effective. That's when drag-over effects are most likely.
(2) The Dom's stub axles are made of rather light steel tubing. If the tundra is wider and its center sits farther out on the stub than the original 'barrow wheels, you'll be putting a higher bending load on those stubs. Replacing them is possible, but kind of a pain since they are welded in place and must be aligned in caster and camber while welding, then fine-tuned while heating them with a welding torch.
I don't know how severe either of these effects would be (other than the "kind of a pain" part). They're worth taking in account, though. Some safe experimenting before "committing aviation" would be wise.
Did you have to modify the axles in any way?