I am definitely filing on time and am going to visit with accountant on options.
The amount I have due on April 15 is 5 figures but I will be able to pay over half maybe 3/4 of it on time. The remainder I should be able to pay in May or June.
You have two practical options and either one should work. You can file the return on April 15th and pay what you can. About 6 - 8 weeks later you'll get an IRS notice requesting the unpaid balance. That should hopefully coincide with your obtaining the necessary funds, so you can then send in the balance due according to the instructions on the notice.
The second option is to file an extension along with a partial payment. Then when the funds are available in late May or June, file the return along with the balance due. In either case there will be interest, and also a late payment penalty of 1/2% per month if the tax owed after 4/15 is greater than 10% of the total tax on the 1040. The advantage to the second option is that if the funds don't become available until several months later than expected, you won't be subject to the ongoing IRS collection notices that will automatically be sent after you file the tax return with less than full payment.
A word of caution on filing extension Form 4868 in this type of situation. You should list on line 4 of the form the proper amount of "estimate of total tax liabilty" so that the balance due on line 6 is greater than the actual "amount you are paying" on line 7. Some people intentionally and incorrectly reduce the line 4 and 6 amounts so the "balance due" amount equals the amount they have available to pay. The risk here is that if the line 4 amount is significantly lower than the actual tax that shows on the the filed tax return, the IRS can later claim the tax was not properly estimated for extension purposes and retroactively disallow the extension, resulting in late filing penalties. This is rare but can happen.
Since you won't have full payment available by April 15th, you presumably won't be able to pay first quarter estimates and will need to catch-up on those later.
You did not mention state taxes, but if any are due I would be inclined to pay off the state first since states in general can be more difficult to deal with than the IRS.