What you've got there should be approximately the same case volume as either a 6.5x54MS or 6.5x55Swede...probably a little less though since the Swift has more taper and is slightly smaller diameter where the shoulder begins.
Did you brew it up yourself or did you buy it? If bought, did you get any data from the previous owner?
From a Contender, I personally wouldn't want to push pressures any higher than 40,000-45,000psi for any cartridge with a case head that large. If the action is hard to unlock, then you're loading too hot and you need to think about backing off.
Since you don't have any pressure measuring equipment that psi limit is going to be a hard guideline to follow. Given that, you'll need to watch your primers and case head expansion to determine a safe pressure level.
Mike Bellm has a nice write-up on how to watch case head expansion in the Contender. His site has lots of other good info on the Contender & Encore actions. A good site to bookmark if you're a T/C shooter.
I know one guy that shoots a .260Rem out of an XP100...a completely different animal since the XP bolt action can handle higher pressures than the T/C's break action. He uses 120gr for deer and was using rifle data the couple times I visited with him at the range. I don't remember what powders he was using or what velocity he was getting though.
The hottest caliber that a friend of mine runs through his Contender(G1) is the 7-30Waters...The Waters specs out at about 40,000psi max with roughly the same case head size as your wildcat.
I would definitely put a bit of money away into the kitty for a chronograph. Otherwise, you're just guessing on your velocities even compared to another guy with a similar wildcat.
Even one of the cheaper ShootinChrony models will work fine and you can get em on sale from Midway, MidSouth, etc for under $100.
I get 2350fps shooting 140gr bullets out of an 18.5" 6.5x55 with a mid-range load. I'd guess you would be 200-250fps under that with a 14" barrel...a little faster with a 120gr but exactly how much I'm not sure.