As others have said you can't use IPs that are already in use. You should use a separate subnet that you can route through (to the other network).
I would suggest using something larger than a /30 block (in case you ever add additional locations) -- a /29 would allow you to expand to 6 locations for example.
I would use something separate from your existing networks (so it stands out) but not adjacent (in case you expand). I would suggest something like 10.99.9.0/29.
If you want to be able to test then you should convert your existing VPN to Tunnel based VPNs first:
This has the advantage that you can then more easily make routes to test than Policy based IPsec VPNs. Do this during a period of downtime as the VPN will be down while you switch to a Tunnel interface in the VPN and then create the routes.
Then configure X2 on each router with 1 IP from your Point-to-Point range:
Arlington X2 = 10.99.9.1 /29
Dallas X2 = 10.99.9.2 /29
Both in the LAN zone, both in Static IP mode.
Connect X2 to the PtP interfaces of your ISP's device (eg. the MPLS). This assumes the link is just a Layer 2 link, no routing is being done by the ISP.
You should then be able to use the Diagnostic Ping (or CLI ping) commands from each router to ping the other router's X2 interface (eg. Arlington should be able to ping 10.99.9.2.
If things are working then you can create a Routing policy to test:
So on the Arlington router:
Source: Some 10.74.1.x test IP in Arlington
Destination: Some 10.74.2.x test IP in Dallas
Gateway: 10.99.9.2 (Dallas' X2 IP -- the other side)
Interface: X2 (the interface Arlington needs to use)
Metric: make lower than whatever you used for your Tunnel VPN metric.
Make a similar route on Dallas router (but swap all the IPs, so from a Dallas IP to the Arlington IP, the Gateway will be 10.99.9.1 this time).
I would suggest using something like LAN Speed Test (even the free version):
To test the speed to/from your test machines over the VPN first and then again after putting the routes in. You should see a speed difference as the traffic goes over the MPLS.
If that works then change the routes so the:
Source is: Any
And the Destination is: The network on the other side (instead of just an IP) -- so from Arlington the Destination would be: 10.74.2.0/24.
Metric: still lower than the VPN Tunnel route.
All the traffic will go over the MPLS for everybody. This can be done lunch time as the interruption is very minimal.
As long as the Zones for X2 are the same as your X0 networks then no Firewall rules should be needed if you have Interface Trust enabled for the LAN Zone. I don't believe you will need to make any changes to NAT as everything will be routed (with respect to the internal networks).
You could then disable the VPN, or leave it and configure Probes on the routes, so that the MPLS' routes are disabled if the probe fails (allowing it to fall back to the VPN. Less useful if it's the same ISP in both cases...)
And yes: backup your configs FIRST, and again after :)